Improve Postfix Evaluation Implementation

[sozelfist]

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 (b2acfd2) to head (03d6e1d).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #790 +/- ## ========================================== - Coverage 95.33% 95.32% -0.01% ========================================== Files 310 310 Lines 22657 22631 -26 ========================================== - Hits 21600 21574 -26 Misses 1057 1057 ```

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[sozelfist]

Can you have a look, @vil02?