Add a placeholder forward buffered ANS coder.

This buffered ANS coder supports coding the symbols in forward (decode)
order. Rather than windowing or growing the buffer, right now this
coder merely asserts that the buffer will never overflow.

This approach should allow ANS to be used as a drop in replacement for
other entropy coders rather than requiring complicated reversal logic
throughout the codebase.

Change-Id: I6689271233d0e22fea94c51950415dad5af96598