Replace ring by argonautica, rand, hmac and sha2

[thespooler]

This replaces all usages of ring by their RustCrypto equivalent, namely rand, hmac and sha2.
This PR also includes #81, replacing pbkdf2 by argonautica, so as to remove ring from Cargo.toml and libs.rs, completely removing any ring dependency.

• I have read the contribution guidelines
• This change has tests (remove for doc only)
• This change has documentation
• Corresponds to issue Replace ring by rust-argon2, rand, hmac and sha2 #83

[thespooler]

Take a look at primitives/generator.rs. Since there is no equivalent to SecretKey in hmac, I did couple the key and hashing selection mechanism in a few places (new, ephemeral, extract and signature). Meaning that adding another option beside Hmac<Sha256> isn't as easy as the signature of Assertion::new(AssertionKind, key) might lead you to believe.
If you have any idea on how to neatly wrap this, let me know.

[HeroicKatora]

Would you mind making the changes independent of the bpkdf2 removal? I still haven't made up my mind on whether introducing a native dependency (libclang) is good, and want to find out why it is even necessary in the used library. Yes, this means that ring won't be fully removed by this PR but that's fine. Better to go slow and steady than rush.

[thespooler]

I will open a new PR to split argonautica and the rest.