Map voices to channels in MIDI output

[Keith OHara]

Is there more to this change, started in commit e5380f2, on the way?

In 2.13.53, I cannot find a way to make midiInstrument have effect.
There is also the problem reported on -bugs  
http://lists.gnu.org/archive/html/bug-lilypond/2011-03/msg00152.html

Maybe the previous workaround to have one channel per voice could be a model ?
 http://lilypond.org/doc/v2.13/Documentation/notation/creating-midi-files

That workaround did allow changing midiInstrument and midiVolume as in
 http://lilypond.org/doc/v2.13/Documentation/notation/controlling-midi-dynamics
if we set those properties in Voice instead of Staff.
-
Keith