argon2: use blake2 crate for round implementation

[tarcieri]

The argon2 crate presently vendors its own implementation of the BLAKE2b round function for the core of the Argon2 KDF implementation:

https://github.com/RustCrypto/password-hashes/blob/58bc6dd/argon2/src/block.rs#L100-L186

Ideally it'd be able to leverage an implementation from the blake2 crate (which would give us a single place to focus on e.g. SIMD optimizations)

[Pjottos]

I don't think this is feasible, since Argon2 uses a modified version of the round function.
See the Argon2 and Blake2 RFCs

[tarcieri]

@Pjottos agreed