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.

I decided to have this file included from config.h, though, since it
is used nearly everywhere.

Fewer modules needed this than I had expected.

These all have a logical header to go in.

or.h should really include only the minimum of stuff from or/*,
common/*, and lib/*.

libtor-encoding is about various ways to transform data to and from
character sequences.

Nothing in Tor has actually called assert() for some while.

Since packed_cell and destroy_cell exist only to be queued, they go
in the same headers as the queues.

I was expecting this to be much worse.