Upgrade to async_executors 0.5?

If we'd like to keep using the async_executors crate, we should eventually upgrade to version 0.5 or later.

Unfortunately, this will require us to increase our MSRV to 1.54, and to upgrade the tor-rtcompat crate a bit.

As an alternative, we could drop async_executors entirely or make it optional: There's not much left that it provides which we don't already wrap or reimplement in tor-rtcompat.