Support processing MIDI messages in parallel

[oubiwann]

Right now, MIDI message processing happens serially -- and sometimes that's what we want.

But it should also be possible to process messages at the same time (more closely mimicking chord play, for instance.

Tasks:

• [x] Add isParallel field to MidiCallGroup
• [x] Update the Erlang midilib to support passing arbitrary metadata to batch messages (https://github.com/erlsci/midilib/issues/2)
• [x] Update undermidi to support parallel calls via changes/additions to the macros include
• [x] Add parsing for these message changes in erl/messages/midi.go
• [ ] Check to see what operations are unsafe for parallel execution
• [ ] Add mutexes, etc., as needed, in midi/midi.go
• [ ] Add a new DispatchParallel in midi/midi.go that puts the body of the forloop in a goroutine

[oubiwann]

Going to finish the rest of this up in #20