Changes to error types and moved some functions into modules

[giarc3]

• Stopped directly wrapping crypto error messages (like aead::Error) for our own errors
• Changes the Display impls for each error to be more verbose/human-readable
• Moved functions from lib.rs into their respective modules (I think they were all v4-related functions)
  • Note that v4::aes is also re-exported into v5::aes
• Made some functions non-pub, as we don't call them externally, hopefully making our API surface easier to understand next time we visit this repo

[github-actions]

File	Coverage	Lines
All files	90%	90%	✅
src/v4/aes.rs	97%	97%	✅
src/lib.rs	4%	4%	❌
src/aes.rs	83%	83%	✅
src/v5/key_id_header.rs	86%	86%	✅

Minimum allowed coverage is 24%

Generated by 🐒 cobertura-action against 40acdee

[coltfred]

One concern about an error.