SipHash implementation with hand inlining the SIPROUND.
To know details about SipHash, see;
"a fast short-input PRF" https://www.131002.net/siphash/
SIPROUND is defined in siphash24.c that can be downloaded from the above
site. Following license notice is subject to change based on the licensing
policy of siphash24.c.
I2P mods: add off/len version
For constant keys see net.i2p.util.SipHash
0.9.5, Moved to net.i2p.crypto and public since 0.9.27