This is the idiomatic way of defining `poll`-based interfaces in
Rust despite introducing a bit of inconvenience.

To work around this inconvenience, we also introduce a `StreamMuxerExt`
trait allows for better integration with async-await style code and
adds a `_unpin` variant to for each `poll` function of the `StreamMuxer`.

Now that the `StreamMuxer` interface has `Pin<&mut Self>` receivers,
we can drop the `Mutex` from the yamux implementation.

Co-authored-by: Elena Frank <elena.frank@protonmail.com>

The muxer is useless after polling `poll_close` to completion, thus
we can consume `self` in the extension trait method which allows us
to use the function within `libp2p_swarm::Connection`.