Implement the 64-bit variant of xxHash.

Matches C in speed, but it looks like SipHash was not the only bottleneck for #11783. 

For security, I'd say it could replace SipHash wholesale, as per [this post](http://fastcompression.blogspot.hu/2014/07/xxhash-wider-64-bits.html).

Tests include the official data and the relevant ones from SipHash.