blake2b: blake2b password hashing algorithm support

This module implements the blake2b password hashing algorithm, bug-for-bug
compatible with the classic OpenBSD implementation. It is not recommended except
in cases of backwards-compatibility with legacy systems.

This is a low-level module which implements cryptographic primitives. Direct use
of cryptographic primitives is not recommended for non-experts, as incorrect use
of these primitives can easily lead to the introduction of security
vulnerabilities. Non-experts are advised to use the high-level operations
available in the top-level [[crypto::]] module.

Be advised that Hare's cryptography implementations have not been audited.
