Refactor Rabin Karp

[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 95.31%. Comparing base (c3da55f) to head (fee6eb4).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #810 +/- ## ========================================== - Coverage 95.32% 95.31% -0.02% ========================================== Files 310 310 Lines 22478 22430 -48 ========================================== - Hits 21428 21380 -48 Misses 1050 1050 ```

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[sozelfist]

Can you have a look, @vil02?