support tree mode

[grexe]

One of the main strength of BLAKE2 is the tree mode, where hashes for chunks of the input can be computed in parallel, as used in: https://github.com/s3git/bt2sum

Please add support for this feature in the Java library.

[jackzhp]

is that called blake2sp? but this implementation is called blake2s though.

[grexe]

as I understand it, it's part of the core BLAKE2 algorithm, as mentioned in the referenced page:

BLAKE2 supports arbitrary-depth trees