Add sounds

Closes https://github.com/nathan/phosphorus/issues/6.

- [x] Sounds
- [x] Ignore ADPCM sounds
- [x] Play sound until done
- [x] Clones play sounds in main sprite
- [x] Numeric sound indices
- [x] Notes
- [x] Instruments (right now just triangle waves)
- [x] Drums
- [x] Rest
- [x] Respect tempo
- [x] Respect volume
- [x] Restrict note range
- [x] Multiple notes at once