Support BLAKE2X

[aead]

BLAKE2X is described in this paper and is an extended version of BLAKE2b / BLAKE2s, which can produce hash values up to 256 GiB for the 64 bit version and 128 GiB for the 32 bit version.
Furthermore BLAKE2X can be used as a DRBG.

[aead]

golang/go#19513

[inkeliz]

This is already supported on golang.org/x/crypto/blake2b, by using NewXOF.

[aead]

@inkeliz Thank you for the hint - I'm aware of this, I've implemented the linked package 😃
See BLAKE2Xb , BLAKE2Xs