instruments: add remaining midi context definitons, remove midi-bug
instruments: place Staff and Voice definitions separately
instruments: reorder commands
instruments: move Staff_performer from Staff to Voice

We move this performer from Staff contexts to Voice contexts
so that there separate MIDI channels will be created for each
voice (even if there are multiple voices on one Staff).

The previous commit with this description (93f25a92e17df6e0b)
didn't actually do anything because it moved Staff_performer
around in \layout context defitions, which don't affect MIDI.
instruments: define a Vocal[Staff|Voice] and derive others from that