Refactor Disjoint Set Union

[sozelfist]

Changes

• replaced reserve_exact() with Vec::with_capacity(), a more idiomatic approach for reserving memory for the nodes vector.
• add doccumentations
• rewrite tests

Checklist:

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

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.32%. Comparing base (bc8d6fa) to head (60fab99).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #802 +/- ## ======================================= Coverage 95.32% 95.32% ======================================= Files 310 310 Lines 22484 22488 +4 ======================================= + Hits 21433 21437 +4 Misses 1051 1051 ```

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