Bring fork of blake2_rfc in as a git subtree

[ebfull]

We do this for other stuff, and I think this should be no exception. It avoids me having to check hashes in PRs like this.

[str4d]

The more likely alternative avenue is to switch to the blake2 crate once RustCrypto/hashes#78 is merged (which I ported from our fork of blake2_rfc).