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()
* ListenerStream unit tests: transport() and listeners()
* Tests for nodes/listeners.rs
* Add a few tests to help illustrate the "drowning" behaviour of busy listeners
* Address grumbles
* Remove non-project specific stuff
* Address grumbles
* Prefer freestanding function
* Provide poll() as a regular method when we can't fail
* Add a CollectionReachEvent that allows accepting
* Add small TODO in handled_node
* Implement Debug on collection events
* Implement Debug for CollectionStream
* Implement Debug for structs in handled_node_tasks
* Attempt to fix compilation errors with stable Rust
* Prove the unwrap
