This panics on error, and we're fine with a panic on misbehavior in
tests.

This lint exists for perf reasons, and this is rarely relevant in
tests.

Using double quoted str is generally cognitively less burdensome.

Found these by disabling the nightly dbg macro special case.  Now, we
have a mechanism for globally adding suppressions to tests, we can use
that instead.

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 #326

This is a defensive API choice to protect against the possibility
that we'll want to add a bunch of other non-config options in
the future.

Closes #350

The new `BootstrapBehavior` enum controls whether an unbootstrapped
`TorClient` will bootstrap itself automatically (`Ondemand`) when an
attempt is made to use it, or whether the user must perform
bootstrapping themselves (`Manual`).

The `lazy-init` example shows how you could write a simple
`get_tor_client()` function that used a global `OnceCell` to share
a Tor client across an entire application with this API.

closes #278