Return input stream from cycle-midi driver

[appsforartists]

This has been something I've wanted to do from the beginning, so I'm surprised I don't see an issue for it.

cycle-midi right now will send any notes it receives to the first instrument. It should also listen to notes from that instrument, and return a stream of them. This will allow a Cycle application to both write notes to an instrument and read notes from them.

[appsforartists]

I wonder if it's safe to presume that the same MIDI instrument is both an input and an output. Right now, we presume that there's only one instrument (output). When we add input support, is it safe to keep this presumption, or do we have to handle input and output separately?