Once we need a PRNG, we'll likely want to change the dev-dependency on the rand
crate to be a real dependency, and use rand::SmallRng as our PRNG.

 * FIXES #24660: https://bugs.torproject.org/24660

On second thought, this is a global initialisation function and
doesn't conceptually have much to do with getting/using randomnesses.

 * FIXES part of #24658: https://bugs.torproject.org/24658

 * FIXES part of #24658: https://bugs.torproject.org/24658

 * FIXES part of #24658: https://bugs.torproject.org/24658

 * ADD new /src/common/crypto_rand.[ch] module.
 * ADD new /src/common/crypto_util.[ch] module (contains the memwipe()
   function, since all crypto_* modules need this).
 * FIXES part of #24658: https://bugs.torproject.org/24658

(cherry picked from commit 5a8cdec3)

Previously, the limit for MAX_PROTOCOLS_TO_EXPAND was actually being applied
in Rust to the maximum number of version (total, for all subprotocols).
Whereas in C, it was being applied to the number of subprotocols that were
allowed.  This changes the Rust to match C's behaviour.

The behaviours still do not match, unsurprisingly, but now we know where a
primary difference is: the Rust is validating version ranges more than the C,
so in the C it's possible to call protover_all_supported on a ridiculous
version range like "Sleen=0-4294967294" because the C uses
MAX_PROTOCOLS_TO_EXPAND to count the number of *subprotocols* whereas the Rust
uses it to count the total number of *versions* of all subprotocols.

There's now no difference in these tests w.r.t. the C or Rust: both
fail miserably (well, Rust fails with nice descriptive errors, and C
gives you a traceback, because, well, C).

The DoS potential is slightly higher in C now due to some differences to the
Rust code, see the C_RUST_DIFFERS tags in src/rust/protover/tests/protover.rs.

Also, the comment about "failing at the splitting stage" in Rust wasn't true,
since when we split, we ignore empty chunks (e.g. "1--1" parses into
"(1,None),(None,1)" and "None" can't be parsed into an integer).

Finally, the comment about "Rust seems to experience an internal error" is only
true in debug mode, where u32s are bounds-checked at runtime.  In release mode,
code expressing the equivalent of this test will error with
`Err(ProtoverError::Unparseable)` because 4294967295 is too large.

Previously, if "Link=1-5" was supported, and you asked protover_all_supported()
(or protover::all_supported() in Rust) if it supported "Link=3-999", the C
version would return "Link=3-999" and the Rust would return "Link=6-999".  These
both behave the same now, i.e. both return "Link=6-999".

During code review and discussion with Chelsea Komlo, she pointed out
that protover::compute_for_old_tor() was a public function whose
return type was `&'static CStr`.  We both agree that C-like parts of
APIs should:

1. not be exposed publicly (to other Rust crates),
2. only be called in the appropriate FFI code,
3. not expose types which are meant for FFI code (e.g. `*mut char`,
   `CString`, `*const c_int`, etc.) to the pure-Rust code of other
   crates.
4. FFI code (e.g. things in `ffi.rs` modules) should _never_ be called
   from pure-Rust, not even from other modules in its own crate
   (i.e. do not call `protover::ffi::*` from anywhere in
   `protover::protoset::*`, etc).

With that in mind, this commit makes the following changes:

 * CHANGE `protover::compute_for_old_tor()` to be
   visible only at the `pub(crate)` level.
 * RENAME `protover::compute_for_old_tor()` to
   `protover::compute_for_old_tor_cstr()` to reflect the last change.
 * ADD a new `protover::compute_for_old_tor()` function wrapper which
   is public and intended for other Rust code to use, which returns a
   `&str`.

It was changed to take borrows instead of taking ownership.

 * REFACTOR `protover::ffi::protover_is_supported_here()` to use changed method
   signature on `protover::is_supported_here()`.

This includes a subtle difference in behaviour to the previous Rust
implementation, where, for each vote that we're computing over, if a single one
fails to parse, we skip it.  This now matches the current behaviour in the C
implementation.

 * REFACTOR `protover::ffi::protover_compute_vote()` to use
   new types and methods.

This includes a subtle difference in behaviour, as in 4258f1e18, where we return
(matching the C impl's return behaviour) earlier than before if parsing failed,
saving us computation in parsing the versions into a
protover::protoset::ProtoSet.

 * REFACTOR `protover::ffi::protover_list_supports_protocol_or_later()` to use
   new types and methods.

This includes a subtle difference in behaviour, as in 4258f1e18, where we return
(matching the C impl's return behaviour) earlier than before if parsing failed,
saving us computation in parsing the versions into a
protover::protoset::ProtoSet.

 * REFACTOR `protover::ffi::protover_list_supports_protocol()` to use new types
   and methods.

This includes differences in behaviour to before, which should now more closely
match the C version:

 - If parsing a protover `char*` from C, and the string is not parseable, this
   function will return 1 early, which matches the C behaviour when protocols
   are unparseable.  Previously, we would parse it and its version numbers
   simultaneously, i.e. there was no fail early option, causing us to spend more
   time unnecessarily parsing versions.

 * REFACTOR `protover::ffi::protover_all_supported()` to use new types and
   methods.

Previously, the rust implementation of protover considered an empty string to be
a valid ProtoEntry, while the C version did not (it must have a "=" character).
Other differences include that unknown protocols must now be parsed as
`protover::UnknownProtocol`s, and hence their entries as
`protover::UnvalidatedProtoEntry`s, whereas before (nearly) all protoentries
could be parsed regardless of how erroneous they might be considered by the C
version.

My apologies for this somewhat messy and difficult to read commit, if any part
is frustrating to the reviewer, please feel free to ask me to split this into
smaller changes (possibly hard to do, since so much changed), or ask me to
comment on a specific line/change and clarify how/when the behaviours differ.

The tests here should more closely match the behaviours exhibited by the C
implementation, but I do not yet personally guarantee they match precisely.

 * REFACTOR unittests in protover::protover.
 * ADD new integration tests for previously untested behaviour.
 * FIXES part of #24031: https://bugs.torproject.org/24031.

This changes `protover::is_supported_here()` to be aware of new datatypes
(e.g. don't call `.0` on things which are no longer tuple structs) and also
changes the method signature to take borrows, making it faster, threadable, and
easier to read (i.e. the caller can know from reading the function signature
that the function won't mutate values passed into it).

 * CHANGE the `protover::is_supported_here()` function to take borrows.
 * REFACTOR the `protover::is_supported_here()` function to be aware of new
   datatypes.
 * FIXES part of #24031: https://bugs.torproject.org/24031

This adds a new type for votes upon `protover::ProtoEntry`s (technically, on
`protover::UnvalidatedProtoEntry`s, because the C code does not validate based
upon currently known protocols when voting, in order to maintain
future-compatibility), and converts several functions which would have operated
on this datatype into methods for ease-of-use and readability.

This also fixes a behavioural differentce to the C version of
protover_compute_vote().  The C version of protover_compute_vote() calls
expand_protocol_list() which checks if there would be too many subprotocols *or*
expanded individual version numbers, i.e. more than MAX_PROTOCOLS_TO_EXPAND, and
does this *per vote* (but only in compute_vote(), everywhere else in the C seems
to only care about the number of subprotocols, not the number of individual
versions).  We need to match its behaviour in Rust and ensure we're not allowing
more than it would to get the votes to match.

 * ADD new `protover::ProtoverVote` datatype.
 * REMOVE the `protover::compute_vote()` function and refactor it into an
   equivalent-in-behaviour albeit more memory-efficient voting algorithm based
   on the new underlying `protover::protoset::ProtoSet` datatype, as
   `ProtoverVote::compute()`.
 * REMOVE the `protover::write_vote_to_string()` function, since this
   functionality is now generated by the impl_to_string_for_proto_entry!() macro
   for both `ProtoEntry` and `UnvalidatedProtoEntry` (the latter of which is the
   correct type to return from a voting protocol instance, since the entity
   voting may not know of all protocols being voted upon or known about by other
   voting parties).
 * FIXES part of #24031: https://bugs.torproject.org/24031

rust: Fix a difference in compute_vote() behaviour to C version.

This implements conversions from either a ProtoEntry or an UnvalidatedProtoEntry
into a String, for use in replacing such functions as
`protover::write_vote_to_string()`.

 * ADD macro for implementing ToString trait for ProtoEntry and
   UnvalidatedProtoEntry.
 * FIXES part of #24031: https://bugs.torproject.org/24031

This adds a new protover::UnvalidatedProtoEntry type, which is the
UnknownProtocol variant of a ProtoEntry, and refactors several functions which
should operate on this type into methods.

This also fixes what was previously another difference to the C implementation:
if you asked the C version of protovet_compute_vote() to compute a single vote
containing "Fribble=", it would return NULL.  However, the Rust version would
return "Fribble=" since it didn't check if the versions were empty before
constructing the string of differences.  ("Fribble=" is technically a valid
protover string.)  This is now fixed, and the Rust version in that case will,
analogous to (although safer than) C returning a NULL, return None.

 * REMOVE internal `contains_only_supported_protocols()` function.
 * REMOVE `all_supported()` function and refactor it into
   `UnvalidatedProtoEntry::all_supported()`.
 * REMOVE `parse_protocols_from_string_with_no_validation()` and
   refactor it into the more rusty implementation of
   `impl FromStr for UnvalidatedProtoEntry`.
 * REMOVE `protover_string_supports_protocol()` and refactor it into
   `UnvalidatedProtoEntry::supports_protocol()`.
 * REMOVE `protover_string_supports_protocol_or_later()` and refactor
   it into `UnvalidatedProtoEntry::supports_protocol_or_later()`.
 * FIXES part of #24031: https://bugs.torproject.org/24031

rust: Fix another C/Rust different in compute_vote().

This fixes the unittest from the prior commit by checking if the versions are
empty before adding a protocol to a vote.

This replaces the `protover::SupportedProtocols` (why would you have a type just
for things which are supported?) with a new, more generic type,
`protover::ProtoEntry`, which utilises the new, more memory-efficient datatype
in protover::protoset.

 * REMOVE `get_supported_protocols()` and `SupportedProtocols::tor_supported()`
   (since they were never used separately) and collapse their functionality into
   a single `ProtoEntry::supported()` method.
 * REMOVE `SupportedProtocols::from_proto_entries()` and reimplement its
   functionality as the more rusty `impl FromStr for ProtoEntry`.
 * REMOVE `get_proto_and_vers()` function and refactor it into the more rusty
   `impl FromStr for ProtoEntry`.
 * FIXES part of #24031: https://bugs.torproject.org/24031

 * ADD new type, protover::UnknownProtocol, so that we have greater type safety
   and our protover functionality which works with unsanitised protocol names is
   more clearly demarcated.
 * REFACTOR protover::Proto, renaming it protover::Protocol to mirror the new
   protover::UnknownProtocol type name.
 * ADD a utility conversion of `impl From<Protocol> for UnknownProtocol` so that
   we can easily with known protocols and unknown protocols simultaneously
   (e.g. doing comparisons, checking their version numbers), while not allowing
   UnknownProtocols to be accidentally used in functions which should only take
   Protocols.
 * FIXES part of #24031: https://bugs.torproject.org/24031

 * ADD new protover::protoset module.
 * ADD new protover::protoset::ProtoSet class for holding protover versions.
 * REMOVE protover::Versions type implementation and its method
   `from_version_string()`, and instead implement this behaviour in a more
   rust-like manner as `impl FromStr for ProtoSet`.
 * MOVE the `find_range()` utility function from protover::protover to
   protover::protoset since it's only used internally in the
   implementation of ProtoSet.
 * REMOVE the `contract_protocol_list()` function from protover::protover and
   instead refactor it (reusing nearly the entire thing, with minor superficial,
   i.e. non-behavioural, changes) into a more rusty
   `impl ToString for ProtoSet`.
 * REMOVE the `expand_version_range()` function from protover::protover and
   instead refactor it into a more rusty implementation of
   `impl Into<Vec<Version>> for ProtoSet` using the new error types in
   protover::errors.
 * FIXES part of #24031: https://bugs.torproject.org/24031.

This will allow us to do actual error handling intra-crate in a more
rusty manner, e.g. propogating errors in match statements, conversion
between error types, logging messages, etc.

 * FIXES part of #24031: https://bugs.torproject.org/24031.

Previously, the limit for MAX_PROTOCOLS_TO_EXPAND was actually being applied
in Rust to the maximum number of version (total, for all subprotocols).
Whereas in C, it was being applied to the number of subprotocols that were
allowed.  This changes the Rust to match C's behaviour.

The behaviours still do not match, unsurprisingly, but now we know where a
primary difference is: the Rust is validating version ranges more than the C,
so in the C it's possible to call protover_all_supported on a ridiculous
version range like "Sleen=0-4294967294" because the C uses
MAX_PROTOCOLS_TO_EXPAND to count the number of *subprotocols* whereas the Rust
uses it to count the total number of *versions* of all subprotocols.

There's now no difference in these tests w.r.t. the C or Rust: both
fail miserably (well, Rust fails with nice descriptive errors, and C
gives you a traceback, because, well, C).

The DoS potential is slightly higher in C now due to some differences to the
Rust code, see the C_RUST_DIFFERS tags in src/rust/protover/tests/protover.rs.

Also, the comment about "failing at the splitting stage" in Rust wasn't true,
since when we split, we ignore empty chunks (e.g. "1--1" parses into
"(1,None),(None,1)" and "None" can't be parsed into an integer).

Finally, the comment about "Rust seems to experience an internal error" is only
true in debug mode, where u32s are bounds-checked at runtime.  In release mode,
code expressing the equivalent of this test will error with
`Err(ProtoverError::Unparseable)` because 4294967295 is too large.