Clean up MIDI stuff and document it

Current scheme of rotating through channels still has audible pitch shifts in it.

Alternatively, I could assign a certain amount of pitch shift to each channel and adjust it by a smaller amount, but then if you play multiple tones near that pitch shift it could still be audible.

Other possibilities:

OpenSoundControl - Just a generic communication protocol, doesn't have any standard way of sending notes, de facto or otherwise
MIDI Tuning Standard (MTS) - lets you define specific scales, but not send arbitrary frequencies?
HD-MIDI - is an old name for MIDI 2.0? I think?
MIDI 2.0 - has a "Note On With Attribute # 3 Pitch 7.9" that allows you to send individual notes with precise frequency? Yay that's what I want! Except it's not actually supported by any synthesizers yet. Boo.
MIDI Polyphonic Expression (MPE) - lets you set pitch bends per-note? That seems good enough, too, but maybe also not supported by anything. 😕

Could MTS be combined with channel rotation to get an improved system?

endolith / just_intonation

Clean up MIDI stuff and document it #3