tor-config: Add HasKind support.

See merge request tpo/core/arti!296

This required a few new ErrorKinds.

This edge-case was there even before the migration of
595fe1ab, but now it's more explicit and ought to be
revisited.

Remove the use of Mutex in channel unused_since timestamp

See merge request tpo/core/arti!293

Temporarily allow failures in the coverage CI.

See merge request tpo/core/arti!295

Right now it's failing on nightly through no fault of our own.

Another set of improvements to the error code

Closes #321

See merge request tpo/core/arti!294

This fixes a compilation error.

Instead of declaring a macro that takes vis as an argument, we now
conditionally declare a macro that applies an appropriate visibility.

There's a long comment explaining the rationale here, along with a
couple of other solutions that don't work.

This is closer to common usage.

(Not that we all agree with common usage, but it's closer to what
people expect.)

This is closer to what we described in Errors.md.

Also, remove the (sometimes private) Result alias: it was only used in
one or two places, and never exposed in public.

This change lets us make TorError's members unconditionally hidden,
and makes our API a little more consistent (since basically nothing
else is a public field).

Hopefully uncontroversial improvements to new Error code

See merge request tpo/core/arti!291

Expire channels that have been unused for too long

See merge request tpo/core/arti!273

These tests turned up a need for using the #[track_caller]
annotation in order to get accurate locations, which is fortunately
stable since Rust 1.46.0.

(By our convention, these errors should say what we were trying to
spawn when the error occurred.)

At least by default, we should have Error be private, and not expose
it as part of our APIs.

To keep functionality in `arti`, I had to add an `ExitTimeout` error
kind.

For interface consistency, I also re-exported ErrorKind and HasError
from `arti_client`.

I had planned to use assert_impl, but that can't check for
the 'static lifetime.

This patch makes only minimal changes in lower-level error types:
we have more refactoring to do.

New error handling proof of concept

See merge request tpo/core/arti!262

This is the first version that builds correctly on our CI.  It's
from back in 2018, so requiring it shouldn't cause any major
problems.

Properly linkify two doc comment xrefs to issues

See merge request tpo/core/arti!290

Fixes these messages:

  warning: this URL is not a hyperlink
   --> crates/arti/src/watch_cfg.rs:115:5
    |
115 | /// https://github.com/notify-rs/notify/issues/165 and
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use an automatic link instead: `<https://github.com/notify-rs/notify/issues/165>`
    |
    = note: `#[warn(rustdoc::bare_urls)]` on by default
    = note: bare URLs are not automatically turned into clickable links

warning: this URL is not a hyperlink
   --> crates/arti/src/watch_cfg.rs:116:5
    |
116 | /// https://github.com/notify-rs/notify/pull/166 .
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use an automatic link instead: `<https://github.com/notify-rs/notify/pull/166>`
    |
    = note: bare URLs are not automatically turned into clickable links

As per
  tpo/core/arti!262 (comment 2772816)

As per
  tpo/core/arti!262 (comment 2772810)

This will get quite large and boxing it here is very convenient.

This also avoids us exposing a large error type to our callers.

The motivation for doing this now is to remove the `#[from]` so we
would spot where operationsl circuit setup failures were handled.
(But it turns out that they are turned into internal errors!)

Perhaps this will want to become a different error type from circmgr
in due course, but for now we simply use a bespoke variant of
TorError.

It will want its own Kind.  The TODO in the HasKind impl marks
this (amongst much else here).