SipHash *is* a cryptographic function, with a 128-bit security level.

However, it is not a regular hash function: a secret key is required,
and knowledge of that key allows collisions to be quickly computed offline.

SipHash is therefore more suitable to be used as a MAC.

The same API as other MACs was implemented in addition to functions directly
returning an integer.

The benchmarks have been updated accordingly.

No changes to the SipHash implementation itself.