feat: move hash functions to dedicated hashing module and add MD5

[AliAlimohammadi]

Description

Adds an MD5 hash function implementation (src/hashing/md5.rs) and moves all existing hash functions out of src/ciphers/ into a new dedicated src/hashing/ module.

Why the restructure: Hash functions are fundamentally different from ciphers — they are one-way and non-reversible. Keeping them in ciphers/ was misleading. This change mirrors how the repository already separates concerns into sorting/, graph/, data_structures/, etc.

MD5 implementation: A pure-Rust implementation following RFC 1321. It processes input in 512-bit blocks across four rounds of 16 operations each, using precomputed sine-derived constants (K table) and per-round bit-shift amounts (S table), producing a 128-bit (16-byte) little-endian digest.

⚠️ Note: MD5 is cryptographically broken and is included for educational/checksum purposes only.

Files changed:

• src/hashing/md5.rs — new MD5 implementation
• src/hashing/mod.rs — new module, re-exports all hash functions
• src/ciphers/mod.rs — removed hashing entries
• src/lib.rs — added pub mod hashing
• Moved: blake2b.rs, sha1.rs, sha2.rs, sha3.rs, hashing_traits.rs → src/hashing/

Type of change

• New feature (non-breaking change which adds functionality)

Checklist

• I ran cargo clippy --all -- -D warnings just before my last commit and fixed any issue that was found.
• I ran cargo fmt just before my last commit.
• I ran cargo test just before my last commit and all tests passed.
• I added my algorithm to the corresponding mod.rs file within its own folder, and in any parent folder(s).
• I added my algorithm to DIRECTORY.md with the correct link.
• I checked CONTRIBUTING.md and my code follows its guidelines.

[AliAlimohammadi]

@siriak, this is ready to be merged.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.00%. Comparing base (d3c9028) to head (5d3b356).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1033      +/-   ##
==========================================
+ Coverage   95.98%   96.00%   +0.01%     
==========================================
  Files         389      390       +1     
  Lines       29520    29605      +85     
==========================================
+ Hits        28335    28421      +86     
+ Misses       1185     1184       -1     

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[Copilot]

Pull request overview

This PR introduces a dedicated hashing/ module to separate one-way hash algorithms from reversible ciphers, and adds a new MD5 implementation alongside existing SHA/BLAKE hashing algorithms.

Changes:

• Added src/hashing/ module and exposed it via src/lib.rs
• Moved existing hash implementations (SHA-1/SHA-2/SHA-3/BLAKE2b + hashing traits) out of ciphers/ into hashing/
• Added a pure-Rust MD5 implementation (plus hex helper) and updated DIRECTORY.md

Reviewed changes

Copilot reviewed 5 out of 10 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/lib.rs	Exposes the new hashing module from the crate root.
src/hashing/mod.rs	Defines the hashing module and re-exports hashing APIs (including MD5).
src/hashing/md5.rs	Adds MD5 + md5_hex with RFC test vectors.
src/hashing/sha1.rs	SHA-1 implementation relocated under hashing.
src/hashing/sha2.rs	SHA-2 family implementation relocated under hashing.
src/hashing/sha3.rs	SHA-3 implementation relocated under hashing.
src/hashing/blake2b.rs	BLAKE2b implementation relocated under hashing.
src/hashing/hashing_traits.rs	Hashing traits + HMAC relocated under hashing.
src/ciphers/mod.rs	Removes hashing modules/re-exports from ciphers.
DIRECTORY.md	Updates directory listing to reflect the new hashing module and MD5.

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