Multi-instrument support

Nobody has claimed this yet.

There could be an `Instrument` interface that is implemented by instrument types such as `RenderedSynthInstrument`, `RealtimeSynthInstrument`, and `RealInstrument`. (There's not a great built-in way to do interfaces in JS, so "interface" is purely conceptual here.)

These instruments would be managed by the `NotePlayer` class.

* `PrerenderedSynthInstrument` would be how the code currently works - pre-render a scale of notes into an audio buffer, then play the buffer piecemeal using multiple voices as notes are requested.

* `RealtimeSynthInstrument` would schedule and play synth notes in real time, the way the `Tone.js` library works by default.

* `RealInstrument` would load in a set of audio files (or an audio sprite) from URL(s). This could allow a ToneMatrix instrument to be backed by recordings of real instruments such as piano.

Start by locating the NotePlayer class and the existing pre-rendered synth path. Read how current notes and voices are managed, then compare the proposed PrerenderedSynthInstrument, RealtimeSynthInstrument, and RealInstrument responsibilities. Done would mean a decided instrument abstraction that NotePlayer can manage across the proposed playback modes.

Written by the indexing model from the issue text.