Midi: Map voices to tracks in MIDI output.

Midi: Map voices to tracks in MIDI output.

With voices mapped to tracks, midi2ly can neatly recreate voices
without needing to do tricky guessing.  Some extra trickery will
be needed to re-assign the voices to the correct staves.

Also, midi channels are optimally re-used: identical midi instruments
accross different staves share the same channel.  This can fix
running out of channels in some cases, where similar instruments
are notated on different staves.