Make update_config only conditionally exported; add semver-status update.

Replace the recapitulation of TorClientConfig fields in ArtiConfig and
instead just have it contain one.  This is part of #374.

The conversions from ArtiConfig back to ArtiConfigBuilder and
TorClientConfigBuilder would need to change, but, since we don't want
them anyway,

No longer impl Deserialize for ArtiConfig.  (As per #371 this will
want to become a private type.)

No longer impl From<ArtiConfig> for ArtiConfigBuilder and
TorClientConfigBuilder.  And abolish tests of that code.

(This all has to be in one commit, because previously
ArtiConfig::tor_client_config used the validated-to-builder config
retcon.)

We perform this operation in a bunch of places, and most of them
use hex::decode().  That's not great, since hex::decode() has to do
heap allocation.  This implementation uses hex::decode_to_slice(),
which should be faster.

(In the future we might choose to use one of the faster hex
implementations, but I'm hoping that this change will be sufficient
to get hex decoding out of our profiles.)

Part of #377.

Previously they returned an Arc, which wasn't necessary unless the
client actually _wanted_ a new Arc.

This would be an API break, except that these functions are marked
'experimental-api', so semver does not apply; nonetheless I've noted
the break in semver_status.md, just in case we care.

Closes #369

The redundant method was a `to_owned` that probably shouldn't have
been called that.  It was only used in one place.

The tests should get tor-linkspec's line coverage up above 90%.

This was a relic of the old, now-unused "caret_enum!" macro.
Removing it gets caret's coverage to 100%.

Yes, technically this is a semver breaker on caret.

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