USB-MIDI device support

[trentgill]

Crow should simultaneously appear as a serial device, and a USB MIDI device to a connected host. This will allow out-of-box usage of crow with any MIDI capable host, specifically making integration into DAWs other than Ableton possible.

The full scripting environment will still be used, so the precise behaviour of the MIDI interop will be reconfigurable.

Defaults will of course be MIDI->CV conversion, but also MIDI->I2C bus conversion (eg. play Just Friends from your DAW, or sequence a hardware synth with the input jacks.

With #339 this could allow auto-synchronization between a USB host and crow's clock module.

[trentgill]

@tehn to write a super basic script example of what the syntax could look like / what features (events etc) would be enabled.

[tehn]

norns midi reference: https://github.com/monome/norns/blob/main/lua/core/midi.lua

basically the same, without vports. to_msg and to_data are key. ie:

function midi.note_on(note, vel, ch)
  midi.send{type="note_on", note=note, vel=vel, ch=ch or 1}
end

where midi.send breaks it down into bytes and forwards to HID-MIDI endpoint.

midi output

--- play a random note on in 1 change
note = 0
input[1].mode("change")
input[1].change = function(state)
  if state==1 then
    note = math.random(127)
    midi.note_on(note)
  else
    midi.note_off(note)
  end
end

midi input

midi.event gets called with midi data, which is first passed through to_msg for clarity. to_msg should have a raw type for match failure.

midi.event = function(e)
  if e.type == "note_on" then
    output[1].volts = midi_to_volt(e.note)
    output[2]() -- bang envelope
  elseif e.type == "cc" then
    if e.cc == 10 then
      output[3].volts = 10/e.value
    end
  end
end

[trentgill]

Consider #404 alternate clock sources if this ever makes it. would be nice to support MIDI input as the clock source, and feels preferable to a custom solution with lua functions etc.

Could be used with Norns quite easily as the MIDI clock system is built-in, rather than having to add custom params for turning on a special 'crow' sync.