Add Serial-over-MIDI communication channel.
Add Serial-over-MIDI communication channel.

WALT commands will also be read from MIDI channel pressure messages, and
sending a MIDI program change message for program number 1 will change
WALT responses to be sent via. MIDI system exclusive messages -- rather
than the USB serial interface (any other program number will disable
this feature).

google/walt,google/walt,google/walt,google/walt