Cdef recorder (#424)

* Add a bitstream writer record/replay abstraction

Allows encoding piecemeal, out of order, or in parallel; a recording
writer can be played back into a master bitstream range encoder.

* Reimplement od_ec_enc and Writer, add record/playback

Flatten od_ec_enc and Writer, as one was a thin wrapper for the other
Reimplement range encoding writer as a Writer trait and encoding backend
Reimplement recording as a second backend to same Writer trait