Flesh out some test code in the RX path to attempt to decode the
RX stream into both frames and commands.

The RX stream stuff is a little .. kooky. I need to figure out
how the stream piecing-together works. That'll come soon.

So:

* Modify the RX bulk code to use an external buffer
* Implement an initial RX stream decoder which doesn't try to
  handle partial / corrupt streams, but instead just handles
  what it can.
* .. frames go nowhere for now
* .. commands get decoded, but they're not yet handled.

This still doesn't yet pop completed RX commands off the RX
stack.  There are also some interesting implications here for
handling notifications versus command responses.