usbd: Implement USB-CDC serial class.

This is a squash of commits from branch feature/usbd_python_cdc which
includes an initial implementation by @hoihu and bug fixes from @linted.

Includes:

- Some usbd device behaviour changes to manage transfers.

- A new reader/writer "Buffer" class designed for providing buffered data
  to/from TinyUSB.

- cdc_example.py opens a REPL on the dynamic USB CDC interface.

- cdc_rate_benchmark.py is an update of the original PyBoard VCP benchmark
  code.

Parts of this work was funded through GitHub Sponsors.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

Author: projectgus

micropython/usbd/__init__.py

@@ -1,4 +1,5 @@
 from .device import get_usbdevice, USBInterface
 from .hid import HIDInterface, MouseInterface
 from .midi import DummyAudioInterface, MIDIInterface, MidiUSB
+from .cdc import CDC
 from . import utils