Change blake2 hash to use blake2b instead of blake2s

[travishathaway]

While working on the JLAP implementation, I discovered that the blake2 hash that rattle_repodata_gateway generates isn't compatible with the ones found in the repodata.jlap files currently being generated by the server (rattler uses blake2s and JLAP uses blake2b).

This pull request switches rattler_repodata_gateway to use blake2b.

Comments and suggestions for improvements welcome.

[baszalmstra]

I thinks its fine to switch the hash type. I also think rattler_digest contains a few things you implemented here or was that not sufficient.

[travishathaway]

@baszalmstra,

I'm guessing I overlooked a few things from rattler_digest. If it's okay to switch hash types, that makes things easier though. I can re-work this pull request to do that instead 👍.

[wolfv]

Thanks!