These metrics are primarily concerned about heap usage and download
size.  I've worked on them for our latest report, and I want to
check them in someplace so we don't forget them the next time we
want to run these.

Now (per discussion on #471) we use one file per crate.

I've tried to be more explicit about the format to use, and what to
add when.

This was anomalous, in that it contains &'static str, rather than a
proper nested error (eg a config::ConfigError, maybe).

But in fact it tursn out it is now not constructed.  The last
construction site was removed a long time ago in
  Use derive_builder for Authority and FallbackDir.

This will help implement #466.

I need this so that I can expose the skew time for the directory
that a circuit will use, when I only have the circuit.

- arti#445 highlighted the lack of good documentation around Arti's
  multiple runtime support, as well as it being difficult to determine
  what runtime was actually in use.
- Improve the documentation to solve the first problem.
- To solve the second problem, make Runtime require Debug (which is
  arguably a good idea anyway, since it makes them easier to embed in
  things), and print out the current runtime's Debug information when
  arti is invoked with `--version`.
- (It also prints out other Cargo features, too!)

fixes arti#445

Relay nicknames are always between 1 and 19 characters long, and
they're always ASCII: That means that storing them in a [u8;19] will
always be possible, and always use less resources than storing them
in a String.

Fortunately, the tinystr crate already helps us with this kind of
thing.

I'm doing this per discussion, so that we can have it be part of the
TorConfig later on, and not break stuff as we change the Mistrust
API to have a builder.

This change, unfortunately, results in a little more internal
complexity and duplicated code in arti and arti-client.  I've marked
those points with TODOs.

This is derived from the environment, not the configuration file: We
might not want to trust the configuration file until we've decided
whether we like its permissions.

And drop the ad-hoc orport() method.  This brings FallbackDir's
orports field in line with our list builder API.

The general semver note in "configuation" seems to cover most of this.

Now that we require Rust 1.56, we can upgrade to AES 0.8.  This
forces us to have some slight API changes.

We require cipher 0.4.1, not cipher 0.4.0, since 0.4.0 has
compatibility issues with Rust 1.56.

This entire struct is config.  This is otiose.

These were omitted in the earlier commits.

In
  tpo/core/arti!462 (comment 2797697)
we decided not to do this.

However, having looked again at the way the FallbackList works, I
think there is a lot of value in making these two things (and anything
else like them[1]) as similar as possible.

[1] At least PreemptiveCircuitConfig.initial_predicted_ports and
NetworkConfig.authorities need the same treatment, and perhaps also
GuardUsage.restrictions (although there is no
GuardRestrictionBuilder).

In the irc discussion I imagined `LogfilesConfigBuilder` as opposed to
`LogfileConfigBuilder` (differing only in the `s`) which would be bad,
but we can use `List` instead.

We do *not* need to abstract away the validated version of the config.
Providing a type alias helps the derive_builder sub_builder DTRT
without needing special overrides.

I have split this commit so that we can drop it, if we conclude it's
not wanted.

Now the network fallbacks configuration wants to Deserialize
a Vec<FallbackDirBuilder>, rather than validated Vec<FallbackDir>.

Methods on FallbackListBuilder are as per
  tpo/core/arti!462 (comment 2797697)
mutatis mutandi for the fact that this struct has only fallbacks in it.