muxers/mplex: Implement AsyncRead and AsyncWrite for Substream

[thomaseizinger]

Description

This aligns the public API of the libp2p-mplex module with the one
from libp2p-yamux. This change has two benefits:

1. For standalone users of libp2p-mplex, the substreams itself are
   now useful, similar to libp2p-yamux and don't necessarily need to
   be polled via the StreamMuxer. The StreamMuxer only forwards to
   the Async{Read,Write} implementations.

2. This will reduce the diff of core/muxing: Redesign StreamMuxer trait #2648 because we can chunk the one
   giant commit into smaller atomic ones.

Whilst keeping master functional, the state of StreamMuxer is now
a bit awkward until we merge #2648. However, given we are already
doing things this way for libp2p-yamux it shouldn't be an issue?

Links to any relevant issues

Will reduce diff in #2648.

Open Questions

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• A changelog entry has been made in the appropriate crates

[thomaseizinger]

Closing because I am going to repush onto rust-libp2p so I can use stacked PRs.

[thomaseizinger]

See #2706.