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.
• More 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.