[noise] Specifying Blake2sp / Blake2bp

[Jason A. Donenfeld]

Hi folks,

I was looking at Samuel's (CC'd) AVX2 optimized implementations of
Blake2 [1] and noticed there wasn't any implementation for Blake2s.
Samuel explained to me that blake2s and blake2b don't naturally
parallelize, which is why the blake2sp and blake2bp variants exist;
these nicely parallelize, so fast implementations are possible. Given
that Noise is pretty hash-heavy, we have good reason to be interested
in fast hash functions. A few questions:

- Should we add the blake2sp and blake2bp variants to the spec? If not, why?
- Are our hash payload lengths sufficient to actually benefit from the
parallelism offered by the parallel variants?

Jason

[1] https://github.com/sneves/blake2-avx2