This makes sure that we're checking for a nonzero port in all cases.

To do this at all neatly, I had to split out `tor-config` from
`arti-config` again, and putting the lower level stuff (paths,
builder errors) into tor-config.  I also changed our use of
derive_builder to always use a common error type, to avoid
error type proliferation.

Part of #125

(We keep routerdescs in the schema, since we don't want _that_ to
fragment.)

Part of #125.

Nothing in arti currently uses this document type.  Eventually it
will be useful for relays and for bridge clients.

I've left the "SHA1 digest of a router descriptor" type available
unconditinoally, however, since it does get used in a few places.

Part of #125.

We don't currently need a couple of the key manipulation features
that we have, since we aren't yet doing relays or onion service
clients.

Part of #125

Solves a name conflict with the existing tor_client create.

Closes #130.

There are some missing parts here (like persistence and tests)
and some incorrect parts (I am 90% sure that the "exploratory
circuit" flag is bogus).  Also it is not integrated with the circuit
manager code.