Improve Longest Common Subsequence Implementation

[sozelfist]

Changes made

• Decompose the longest_common_subsequence to use helper functions
• Rewrite tests using macro
• Naming variables and functions descriptively
• Add docstrings

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 95.07%. Comparing base (7833c0c) to head (7c9685b).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #771   +/-   ##
=======================================
  Coverage   95.06%   95.07%           
=======================================
  Files         309      309           
  Lines       22796    22803    +7     
=======================================
+ Hits        21672    21679    +7     
  Misses       1124     1124           

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[sozelfist]

Let's have a look, @vil02