Move Entropy methods to lossless.c

Move all the Entropy evaluation methods to lossless.c (from histogram.c).
There's slight difference in the way entropy is computed for evaluating
entropy in prediction methods and histogram (literal) for huffman trees.
Plan (later) to merge few (static) methods and reduce the code size.

This change has no impact on the compression speed/density.

Change-Id: Ife3d96a3c4a8d78a91723d9e0a8d1b78c0256a15