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

Rather than looking up a tuple in a dict, this should be using `packbits` as I have shown. [My comment](https://github.com/scikit-learn/scikit-learn/pull/3304#discussion_r15678118) was also on this line, but for some reason github has made it invisible, despite the code not having changed :(