This commit won't build yet -- it just puts everything in a slightly
more logical place.
The reasoning here is that "src/core" will hold the stuff that every (or
nearly every) tor instance will need in order to do onion routing.
Other features (including some necessary ones) will live in
"src/feature". The "src/app" directory will hold the stuff needed
to have Tor be an application you can actually run.
This commit DOES NOT refactor the former contents of src/or into a
logical set of acyclic libraries, or change any code at all. That
will have to come in the future.
We will continue to move things around and split them in the future,
but I hope this lays a reasonable groundwork for doing so.
