Refactor is_subsequence Implemenation

[sozelfist]

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 95.38%. Comparing base (1ef9bcb) to head (8669be3).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #819      +/-   ##
==========================================
- Coverage   95.38%   95.38%   -0.01%     
==========================================
  Files         312      312              
  Lines       22645    22644       -1     
==========================================
- Hits        21601    21600       -1     
  Misses       1044     1044              

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[sozelfist]

Can you have a look, @vil02?