usbd: Major cleanup, refactor.

- Add micropython-lib 'usbd' package (provisional).
- Update midi implementation a bit.
- Rearrange code to work with package structure
- Convert docstrings to regular comments to save flash.

Author: projectgus

micropython/usbd/__init__.py

@@ -0,0 +1,4 @@
+from .device import get_usbdevice, USBInterface
+from .hid import HIDInterface, MouseInterface
+from .midi import DummyAudioInterface, MIDIInterface, MidiUSB
+from . import utils