Implement XXHash64

https://code.google.com/p/xxhash/source/browse/trunk/xxhash.c (r35)

Benchmark results vs Murmur3_128:

* Murmur3_128

1MB buffer   4,195 ± 59 MB/s
single long  2,220 ± 22 MB/s

* XXHash64

1MB buffer   10,518 ± 107 MB/s
single long   2,825 ±  41 MB/s
Use xxhash64 for computing Slice hash code

Benchmark:

before   2,386 ± 34 MB/s
after   10,580 ± 98 MB/s