blake2s: implement BLAKE2X

This CL implements BLAKE2X, a extensible-output functions (XOF) on top of BLAKE2s.
BLAKE2x is described in https://blake2.net/blake2x.pdf

Change-Id: I20838811e270f4767b8bd5e239a1fe1fb994fe1c
Reviewed-on: https://go-review.googlesource.com/38054
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>
6 files changed