Implementation of SipHash 2-4

See: http://131002.net/siphash/

Consider this as a main "general-purpose" hash for all hashtables: it runs at good speed (competitive with spooky and city) and permits strong keyed hashing. Key your hashtables from a strong RNG, such as rand::Rng.

Although the SipHash algorithm is considered to be cryptographically strong, this implementation has not been reviewed for such purposes. As such, all cryptographic uses of this implementation are strongly discouraged.