Add hash function BLAKE2s defined in RFC 7693.

On amd64/SSE4:
name       time/op
Write64-4    125ns ± 0%
Write1K-4   1.64µs ± 1%
Sum64-4      152ns ± 0%
Sum1K-4     1.67µs ± 1%

name       speed
Write64-4  508MB/s ± 0%
Write1K-4  624MB/s ± 1%
Sum64-4    420MB/s ± 0%
Sum1K-4    612MB/s ± 1%

On amd64/SSSE3:
name       time/op
Write64-4    172ns ± 0%
Write1K-4   2.42µs ± 0%
Sum64-4      172ns ± 0%
Sum1K-4     2.42µs ± 0%

name       speed
Write64-4  370MB/s ± 0%
Write1K-4  423MB/s ± 0%
Sum64-4    370MB/s ± 0%
Sum1K-4    422MB/s ± 0%

Change-Id: I5a17cc32d8f17d43db8b1d879af6ab68b44d630c
Reviewed-on: https://go-review.googlesource.com/30913


Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>