[WIP] Locality Sensitive Hashing for approximate nearest neighbor search(GSoC)

I am not sure I understood this comment. Do you mean that it returns an array, where _find_longest_prefix_match uses just the size of this array? 
One call to bisect_left gives the location of the first match; since we care only about whether there are 0 or more than zero matches, I agree that could suffice (good point). This would involve a little code duplication, and I'm not sure this optimization is to a bottleneck, but worth checking.