`tokio::sync::Mutex` support

[johannescpk]

Great work on the crate! Not sure how feasible async support generally for this approach is, but being able to use tokio's Mutex would be nice.

[bertptrs]

Supporting tokio's mutexes would be nice, but like you said, it can't really work with the current design. I'm using thread-local variables to trace a single thread of execution and track what mutexes it holds. As soon as mutex guards start moving across threads, this does not work anymore. A similar but opposite thing came up with #2 as threads don't even exist in a no_std context.

There should be other ways of handling this, and I'm open to suggestions on how to do so.