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

Alternative DirProvider setup

See merge request tpo/core/arti!347

See !352

Disable clippy::clone_on_ref_ptr

See merge request tpo/core/arti!352

chutney/setup: Wait for port 9150 to open after launching arti

Closes #367

See merge request tpo/core/arti!370

Refactor multiple runtime support in order to clean up documentation

Closes #326

See merge request tpo/core/arti!364

Conflicts:
	crates/arti-hyper/examples/hyper.rs

Trivially resolved.  Hyper example still works.

Fixed conflict in crates/arti-client/src/lib.rs as per tree
from
  tpo/core/arti!364 (comment 2782166)
ie 13e55b8d

This is not a replacement for a real startup protocol, but it may
help repair our CI.

Implement TLS in arti-hyper

See merge request tpo/core/arti!355

add some error to retry_error instead of dropping it

See merge request tpo/core/arti!368

remove usage of dbg!(..)

Closes #368

See merge request tpo/core/arti!367

it's making rust-nightly job fail

The *earlier* versions of these crates pull in *dependencies* that
violate our MSRV policy by requiring a *later* version of Rust.

Empirically, env_logger 0.5.4 would be enough, but practice here seems
to be to just say "cargo update".

This makes using the `PreferredRuntime` the first-class option inside
`arti-client`, freeing users who don't want to think about runtimes from
having to do so.

`TorClient::create_unbootstrapped` and `builder` now automatically
use this runtime, leaving only `builder_custom` for users who wish to
manually specify a runtime.

This lets us clean up the docs a lot: mentions of using custom runtimes
are now relegated to nearer the end of the crate-level documentation,
and we mostly just link to `tor_rtcompat`'s docs to explain more there.
Instead, we take some more time to explain how you use the builder API
to create clients synchronously.

Other doc cleanups included getting rid of the explanation of `TorAddr`
in the main crate-level doc; this is already well-documented elsewhere,
and is something users should discover organically later.

fixes arti#326

tor-circmgr: take_action: Handle Cancelled from the oneshot

Closes #365

See merge request tpo/core/arti!363