v0.1.0 initial beta

This works well enough that I expect other people may be able to run it without difficulty. So far, I have only tested with a Trinket M0 (SAMD21) playing USB MIDI to GarageBand on macOS, but I'm hoping it will work just as well in other configurations.

Things that work:

1. Plaintext music notation parsing (creates an array of MIDI events)
2. MIDI event playback with reasonably low-latency timing
3. Debug messages to measure free memory, CPU use, and playback latency

Things that don't work:

1. The music notation parser does not implement rests.
2. Currently, all voices use treble clef ("C" is MIDI note 60). It would be nice to have two voices use bass clef.