Added MIDI IO driver from Yarns and midi.h from stmlib:

https://github.com/pichenettes/eurorack/blob/master/yarns/drivers/midi_io.cc
https://github.com/pichenettes/eurorack/blob/master/yarns/drivers/midi_io.h
https://github.com/pichenettes/stmlib/blob/master/midi/midi.h