Spooky Hash V2 C-port

Spooky Hash is a very good non-cryptographic 64-bit hash function by Bob Jenkins. It is in the public domain and was originally published in C++ here.

A C port for the first version was created by Andi Kleen and published here. It was subsequently improved by Ziga Zupanec.

I updated the port to SpookyV2 (Spooky V1 contains a mistake that degrades its characteristics somewhat) and cleaned it up a bit.