Faster/lower RAM kmer hash

[iqbal-lab]

I don't know the exact RAM usage by our kmer hash, but for future reference, we could replace it with https://github.com/rob-p/BooM which is a minimal perfect hash map based on Rayan's BBhash - uses about 3bits per element apparently. So for human, if we want to store 13mers, we need to store 66 million kmers--->this would only take 25Mb

[ffranr]

Closing in favor of duplicate: #71