blake2s: add function

This tries to be as similar as possible to the blake2b implementation.
If you diff the two, you'll see just how little needed to be changed.
The test vectors come from the .txt in the reference implementation or
are generated with the reference implementation.

Fixes: #351
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

Author: cosmycoder