extract tor_async_utils::oneshot into ::oneshot-fused-workaround

Having this in the tor-async-utils crate prevents us from doing both of the following without introducing a circular dependency:

• using it in tor-rtmock (which we currently do, particularly in tests).
• using tor-rtmock to test things in tor-async-utils. We don't do this yet, but it is generally sensible to do so. In particular we want to move the stream_peak module there, which is currently tested with tor-rtmock.

Moving this into its own crate avoids this circular dependency.