Add siphash24 module

[stevenroose]

Based on the siphasher crate (https://github.com/jedisct1/rust-siphash/) that is a copy of the implementation in std::hash.

[apoelstra]

Is there a significant performance increase from using unsafe code rather than just letting byteorder do the endianness conversions?

[apoelstra]

Also, is there another implementation we can test against? Or some test vectors?

[stevenroose]

To be honest, I have no idea about the implementation internals. I just took the one from the siphasher package. I also took their unit tests, but I had to adapt them since they were using the std::hash trait or something.

About more unit tests, I will look for some in other languages that can be verified line per line because I understand the ones I have aren't verbatim from siphasher and that might sound phishy :)

[stevenroose]

@apoelstra hehe, I noticed that the test vectors we have are actually the same ones as the reference implementation has: https://github.com/veorq/SipHash/blob/master/vectors.h#L3

I think the siphasher people made a variation on that (so they could use the Hash trait) and I coincidentally reverted their variation back to the original vectors. It's a series of 64 hashes of the series [], [0], [0, 1], [0, 1, 2], ...

[apoelstra]

Looks good. Did not verify the test vectors