optimized histogram
optimized histogram

new version easier to vectorize
leads to smaller code and faster execution
notably at the last recombination stage
(basically, fixed cost).

Assembly inspected with godbolt

On my laptop, with `-mavx2` :
2K block : 1280 MB/s -> 1550 MB/s
8K block : 1750 MB/s -> 1860 MB/s

Cyan4973/FiniteStateEntropy