Cleanup/misc cleanup Suggested by David

[ounsworth]

Issue Link

closes #104

Summary

The existing SHA2 / SHA3 primitives are exposed at the root of the crate as:
bouncycastle_sha2::{SHA224, SHA256, ..,}

I have now put the hmac and hkdf primitives in sub-mods so as not to clutter the roots of the crates' namespaces.
bouncycastle_sha2::hmac::{HMAC_SHA224, HMAC_SHA256, ..,}
bouncycastle_sha2::hkdf::{HKDF_SHA256, HKDF_SHA512, ..,}
bouncycastle_sha3::hmac::{HMAC_SHA3_224, HMAC_SHA3_256, ..,}

rebase this on top of the current (rolled back) release/0.1.3alpha. Probably the easiest way will be to make a new clean branch and merge in each commit one-by-one.

Scope and Risk

Flips the cargo.toml dependency order so that sha2 / sha3 are the user-facing crates, and hmac / hkdf are utility crates. This could affect some packaging stuff that depends on this.

Validation

Tests and docs updated to match. I manually edited through the doc updates.

AI Usage Statement

Did you use AI in creating this pull request:

• No
• Yes, indirectly - no submitted code was generated by AI (e.g., answering questions, performing a review, suggestions, etc.)
• Yes, trivial code changes were generated by AI (e.g., autocompletion of a single line, reformatting, or spell-checking)
• Yes, non-trivial code changes were generated by AI

Assisted-by: Claude Code:claude-opus-5