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.
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
Nick Mathewson