Previously the code would do stuff like

```
    schedule = RetrySchedule::new(INITIAL_DELAY);
```

which is needlessly verbose, since the schedule already keeps track
of its initial delay.

This is the logical place for it, I think: the GuardMgr's job is to
pick the first hop for a circuit depending on remembered status for
possible first hops.  Making this change will let us streamline the
code that interacts with these objects.

The various background daemon tasks that `arti-client` used to spawn are
now handled inside their respective crates instead, with functions
provided to spawn them that return `TaskHandle`s.

This required introducing a new trait, `NetDirProvider`, which steals
some functionality from the `DirProvider` trait to enable `tor-circmgr`
to depend on it (`tor-circmgr` is a dependency of `tor-dirmgr`, so it
can't depend on `DirProvider` directly).

While we're at it, we also make some of the tasks wait for events from
the `NetDirProvider` instead of sleeping, slightly increasing
efficiency.

We now check the handshake certificates unconditionally, and only
report them as _expired_ as a last resort.

(Rationale: if somebody is presenting the wrong identity from a year
ago, it is more interesting that they are presenting the wrong ID
than it is that they are doing so with an expired cert.

We also now report a different error if the certificate is expired,
but its expiration is within the range of reported clock skew.

(Rationale: it's helpful to distinguish this case, so that we can
blame the failure on possible clock skew rather than definitely
attributing it to a misbehaving relay.)

Part of #405.

NETINFO cells, which are sent in every handshake, may contain
timestamps.  This patch adds an accessor for the timestamp in the
Netinfo messages, and teaches the tor-proto code how to compute the
minimum clock skew in the code.

The computation isn't terribly precise, but it doesn't need to be:
Tor should work fine if your clock is accurate to within a few
hours.

This patch also notes a Y2038 problem in the protocol: see
torspec#80.

Part of #405.

This commit refactors the dirclient error type into two cases:
errors when constructing a circuit, and errors that occur once we
already have a one-hop circuit.  The latter can usually be
attributed to the specific cache we're talking to.

This commit also adds a function to expose the information about
which directory gave us the info.

We already have the ability to get peer information from ChanMgr
errors, and therefore from any RetryErrors that contain ChanMgr
errors.

This commit adds optional peer information to tor-proto errors, and
a function to expose whatever peer information is available.

It'll soon more convenient to pass in FallbackDirs as a slice of
references, rather than just a slice of FallbackDirs: I'm going to
be changing how we handle these in tor-dirmgr.

Closes #415

This should save around 1MB per consensus, since every relay has a
'protocols' lines, but there are only a few distinct possibilities
for such a line.

Closes #385.

When the version is a Tor version, we can just parse it; otherwise,
we can intern it.  This shrinks GenericRouterStatus and avoids a lot
of extra help allocations.

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