For proposal 228, we need to cross-certify our identity with our
curve25519 key, so that we can prove at descriptor-generation time
that we own that key.  But how can we sign something with a key that
is only for doing Diffie-Hellman?  By converting it to the
corresponding ed25519 point.

See the ALL-CAPS warning in the documentation.  According to djb
(IIUC), it is safe to use these keys in the ways that ntor and prop228
are using them, but it might not be safe if we start providing crazy
oracle access.

(Unit tests included.  What kind of a monster do you take me for?)