* Rename Transport::RawConn to Output
* Remove AsyncRead + AsyncWrite bound on Transport::Output
* UpgradedNode now always implements Transport
* Add and tweak modifiers for Transport and ConnectionUpgrade
* Secio upgrade now returns the pubkey in its output
* Add upgrade::apply
* Add Transport::and_then
* Rework the swarm
* Rustfmt
* Fix concerns
Currently, connection substreams are added to
`connection_reuse::Shared::active_connections`, but never removed. This
is not least because the `StreamMuxer` trait defines its inbound and
outbound substream futures to always yield a substream and contains no
provision to signal that no more substreams can be created, which would
allow client code (e.g. `ConnectionReuse`) to detect this and purge its
caches.
This PR defines the `StreamMuxer` trait to optionally yield
inbound/outbound substreams and changes `libp2p-mplex` to handle
stream EOFs by marking the underlying resource as closed.
`ConnectionReuse` will remove stream muxers from its active connections
cache if a `None` substream is returned.
