Add new algorithm `BLAKE2b` to `#load_hash`

[odiferousmint]

Hello.

Since it is a compile-time directive, you cannot extend it yourself from the source code (I wonder if this would ever be possible though).

I would like to see BLAKE2b256 (and/or BLAKE2b512) added to the list of supported hash algorithms.

Some useful links:

• https://github.com/BLAKE2/BLAKE2
• https://github.com/jart/cosmopolitan/blob/master/libc/str/blake2.c (BLAKE2b256)

There are lots of self-contained C implementations of this algorithm in the wild anyways.

[Kelimion]

Will look into this for the December release.