Move cell encryption/decription into the RelayCell class

"Less is more."

Dave's pull request for ticket #27112 refactored our RelayCell into a
BaseRelayCell and RawRelayCell class. Think I see what he's going for,
and this was a step forward in some respects. But honestly added a lot
of code and complexity without functionally doing anything new.

Going with a hybrid of our approaches that integrates the part I think
we agree on. Namely, simplifying our Circuit's send() method by moving
cell encryption/decription down into the RelayCell class.

This is a much, *MUCH* smaller diff than what Dave proposed. Happy to
discuss integrating more of the pull request, but lets start with
the part I think we agree on to give us a common point to work from.