[cm/hashed/] with BLAKE2b in [cm/hashed/Merkle]-tree mode.

BLAKE2b-512 is initialised with either "LEAF" or "NODE" keys,
instead of using "0x00"/"0x01" constants prepending to the
data. Although BLAKE2 has ability to set tree-hashing parameters on
its own, many implementations do not provide necessary API for that.

"blake2b-merkle" algorithm identifier is used.
