Implement incremental version of SipHash 2-4 and HalfSipHash 2-4

Implement incremental version of SipHash 2-4 and HalfSipHash 2-4

When inserting items into hashtables (hashmaps), we might have a
fragmented key (as an example we might want to hash DNS name + class +
type).  We either need to construct continuous key in the memory and
then hash it en bloc, or incremental hashing is required.

This incremental version of SipHash 2-4 algorithm is the first building
block.

As SipHash 2-4 is often used in the hot paths, I've turned the
implementation into header-only version in the process.