Misc cleanup (review by David)

[ounsworth]

• Add the preference for squash merges to CONTRIBUTING.md
• Make sure CLAUDE.md is pointing to CONTRIBUTING.md
• Move all the name str's into the Factory crate.
• Move the typedefs for HKDF_SHA256 and HKDF_SHA512 to the SHA2 crate.
  • Fix the docs ... probably that just means putting a docstring on those two typedefs that points back at the bouncycastle_hkdf crate docs.
  • Ditto for HMAC.

[ounsworth]

David had suggested:

• Move all the name str's into the Factory crate.

Currently, each algorithm struct is required to give itself a name in order to impl the Algorithm trait:

/// Metadata about a cryptographic algorithm.
pub trait Algorithm {
    /// String name for the algorithm, used consistently across the library.
    const ALG_NAME: &'static str;
    /// Maximum security strength supported by the algorithm.
    /// In other words, this algorithm can produce outputs up to this security strength,
    /// but may produce outputs with lower security strength, for example, if asked to truncate.
    const MAX_SECURITY_STRENGTH: SecurityStrength;
}

At the moment, I think Factory is the only thing that consumes these string names, but my intuition is to leave this the way it is because other things beyond Factory could use these in the future.

So: Won't Fix