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