Refactor Merge Sort Implementation

[TruongNhanNguyen]

Changes

• Add docstring and remove redundant comments
• Use for loop over a range with step instead while loop (nested) in bottom_up_merge_sort
• Parametrize tests using macro

Checklist:

• I ran bellow commands using the latest version of rust nightly.
• 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 COUNTRIBUTING.md and my code follows its guidelines.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.99%. Comparing base (135ae03) to head (e570b66).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #747      +/-   ##
==========================================
- Coverage   95.02%   94.99%   -0.03%     
==========================================
  Files         303      303              
  Lines       22563    22458     -105     
==========================================
- Hits        21440    21335     -105     
  Misses       1123     1123              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.