Add shutdown functionality to `NodeStream`.
Add `NodeStream::shutdown` to allow triggering the shutdown process,
and `NodeStream::poll_shutdown` as the internal way to drive any
potential shutdown to completion.
Update the `StreamMuxer` trait.
- `read_substream`, `write_substream` and `flush_substream` now return `Poll` instead of `Result`.
- A new `Shutdown` enum allows for half-closing of substreams and is used in `shutdown_substream`.
- `close_inbound` and `close_outbound` have been merged into `shutdown` which takes a `Shutdown` parameter to allow closing only one direction.
- Add a new `flush_all` method to allow flushing after a series of actions (e.g. multiple `shutdown_substream`).
W.r.t. flushing the general idea is that normal use drains buffers over time. Shutting down a substream does not imply flushing, so can be followed by `flush_substream` or (if multiple substreams are to be shut down) a single `flush_all`. Shutting down the muxer itself proceeds likewise, i.e. `shutdown` followed by `flush_all`.
* Add unit tests for core::nodes::NodeStream
* Move DummyMuxer to core/tests
* Address grumbles
* Impl Debug for SubstreamRef<P>
* Add test for poll()
* Don't need to open a substream
* pretty printer test
* More tests for NodeStream poll()
* Remove contentious test of Debug
* Rewrite the swarm
* Small improvement to Debug of ListenersStream
* Fix Swarm::Replaced never being produced
* Fix logic problem when reaching a node in swarm
* Small comment in swarm
* Add closed_multiaddr to Replaced event
* Add address to NodeClosed and NodeError
* Fix concerns
* Remove StreamMuxer::boxed
