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 #2648 because we can chunk the one
giant commit into smaller atomic ones.

Co-authored-by: Max Inden <mail@max-inden.de>

v0.46.0 is not yet released, thus no need to bump to v0.47.0.