Move the HuffmanCost() function to dsp lib

This is to help further optimizations.
(like in https://gerrit.chromium.org/gerrit/#/c/69787/)

There's a small slowdown (~0.5% at -z 9 quality) due to
function pointer usage. Note that, for speed, it's important
to return VP8LStreaks by value, and not pass a pointer.

Change-Id: Id4167366765fb7fc5dff89c1fd75dee456737000
Merge "Move the HuffmanCost() function to dsp lib"