Update sfx for custom instruments
Pico-8 0.1.11 added custom instruments, the ability to use sfx 0-7 as waveforms in other sfx (practically speaking for music patterns). This is implemented in .p8 using the high bit of the waveform nibble (8-F == instruments 0-7) and in memory and .p8.png using the unused high bit of the second byte of a note.

sfx.py's comment needs updating, and this unused bit needs to be parsed, written, and given an accessor. Thankfully, this is backwards compatible with older carts, which always sets this bit to 0, so I don't think we need a format version number check.