The Zeroize implementation and testing got out of hand here. I will improve on that later. See RustCrypto/utils#652, RustCrypto/utils#699 and RustCrypto/utils#700 on progress how we can improve on that.

• Move de/serializing checks for keys to the KeGroup implementation (X25519 still outstanding).
• Removed separate Hash associated type in CipherSuite and now the one from OprfGroup is used.
• Removed P-256 implementation and replaced with generic one over RustCrypto traits.
• Use ZeroizeOnDrop trait where appropriate.
• Removed Key type and instead use type provided by KeGroup.
• Change back permissions for all files to 644 (probably messed up by me because I was using a MacBook).
• Removed documented experimental warning for P-256 🎉.
• Consolidated some CI runs and added more feature combinations to test.
• Remove generic from SlowHash and move it to the method instead.
• Updated MSRV to 1.57 because of dependencies on crypto-bigint.
• Fix DeriveKeyPair implementation using the wrong group for HashToField.

Issues introduced 😅:

• Some places ZeroizeOnDrop had to be written by hand because the derive proc-macro doesn't support GenericArray.
• Can't update to curve25519-dalek v4.0.0-pre.2 because it constrained zeroize to <=1.4. See Update curve25519-dalek requirement from =4.0.0-pre.1 to =4.0.0-pre.2 voprf#64.
• Had to pull in curve25519-dalek v3 optionally for x25519-dalek because the newest version doesn't use v4. See Update dependencies dalek-cryptography/x25519-dalek#87.

Fixes #254.
Fixes #256.
Fixes #258.
Fixes #267.