Change the way image comparison is made from a piece-by-piece
approach using tiles to a compute-everything-in-one-run. Introduce
the concept of score to find the most connected images. Implement
the new approach in C with SIMD instructions to get a
decent speed even for a large image set.

Change the output size of signature 2 from 256 bits to 64 bits to
to equal to the other hash types.