Khaled Mammou authored Sep 19, 2018 and David Flynn committed Nov 01, 2018

This commit provides an m-ary entropy coder based on a fixed-size
dictionary with periodic updates, a cache of recently used symbols
(updated using an LRU eviction policy), and a fallback direct binary
coding of any unhandled symbols.

NB: the proposed version used a context with a halving period
(max_count) of 64 symbols.  However, this conflicts with another
adoption (512 symbols), and a wholesale replacement of the arithmetic
codec and context model.  To resolve the conflict, the existing halving
period (128) is used.