Entropy coder clean-up.

This simplifies a good bit of the error handling, and should make it
 impossible to overrun the buffer in the encoder or decoder, while
 still allowing tell() to operate correctly after a bust.
The encoder now tries to keep the range coder data intact after a
 bust instead of corrupting it with extra bits data, though this is
 not a guarantee (too many extra bits may have already been flushed).
It also now correctly reports errors when the bust occurs merging the
 last byte of range coder and extra bits.

A number of abstraction barrier violations were cleaned up, as well.
This patch also includes a number of minor performance improvements:
 ec_{enc|dec}_bits() in particular should be much faster.

Finally, tf_select was changed to be coded with the range coder
 rather than extra bits, so that it is at the front of the packet
 (for unequal error protection robustness).