Replace pbkdf2 with argon2 via rust-argon2

[thespooler]

This is an alternative to #81 by using rust-argon2 instead of argonautica.
This avoids requiring clang as a dependency.

About argonautica, there's also argon2rs and rust-argon2. From what I see, argon2rs seems decent, pure rust and benchmarked. It doesn't do argon2id, only i or d variant and last commit is from 2017 and the maintainer is MIA from what I can glean from open issues.
rust-argon2 has recent commits and seems alive. Also, the talks on RustCrypto/password-hashes#1 might be of interest. They seem to come to a similar conclusion.

Related issues

• Replace ring by rust-argon2, rand, hmac and sha2 Replace ring by rust-argon2, rand, hmac and sha2 #83

[HeroicKatora]

I like this more than the weird build dependency. This means I finally got around to reviewing the actual crypto involved. Some issues there, it really shows how a good interface can make a crypto API more secure.

[thespooler]

Since I rebased this off of next, do you want me to also include the actual removal of ring from cargo.toml and lib.rs?

[HeroicKatora]

That seems like proper usage of ad now, I was expecting something slightly different in the untangling of functional code.

[HeroicKatora]

Since I rebased this off of next, do you want me to also include the actual removal of ring from cargo.toml and lib.rs?

Not yet.

[thespooler]

Fixed match errors while checking

[thespooler]

The impl From<argon2::Error> for RegistrarError and From<Utf8Error> seems overkill at first glance, but it does make the intent of the code in check() more obvious.

[thespooler]

Since I rebased this off of next, do you want me to also include the actual removal of ring from cargo.toml and lib.rs?

Not yet.

How about now? ;)

[HeroicKatora]

Separate PR, sure 👍