Add a generic CDF decoding routine.

This decodes a value encoded with ec_encode_bin() without using any
 divisions.
It is only meant for small alphabets.
If a symbol can take on a large number of possible values, a binary
 search would be better.

This patch also converts spread_decision to use it, since it is
 faster and introduces less rounding error to encode a single
 decision for the entire value than to encode it a bit at a time.