Added XXH128 and XXH256

Both XXH128/XXH256 are implemented in the spirit of XXH64, with minor tweaks to constants and the final mixin (to pass smhasher tests). They both pass all smhasher tests. Speedwise, they have the same performance as XXH64 for longer keys, and for shorter keys, they are slightly slower, due to more instructions for the final mix.
Hmm, I honestly believed I was up to date. My git skills are poor, I must admit.