Make the RANS implementation operate on cumulative distribution
functions rather than individual probability distribution functions.
CDFs have shown themselves more flexible to work with.

Reduces decoding memory usage from scaling O(num_distributions *
symbol_resolution) to O(num_distributions).

No bitstream change. This is an purely implementation change.

Change-Id: I4e18d3a0a3d37a36a61487c3d778f9d088b0b374