Refactor palindrom checking algorithm

[bgdnrvsky]

Description

• Use simpler approach to check if the string is a palindrome
• Add unit test for empty palindrome

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 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 95.07%. Comparing base (45edeb9) to head (7874b83).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #778 +/- ## ========================================== - Coverage 95.08% 95.07% -0.01% ========================================== Files 309 309 Lines 22810 22773 -37 ========================================== - Hits 21689 21652 -37 Misses 1121 1121 ```

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