Refactor NQueens implementation

[TruongNhanNguyen]

Changes

• Rewrite NQueensSolver in OOP style
• Use n_queens_solver method to hide internal state of NQueensSolver object
• Add Rust docstrings
• Write parametrized tests, add 6x6 case

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 95.04%. Comparing base (c502484) to head (da01917).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #732 +/- ## ========================================== - Coverage 95.04% 95.04% -0.01% ========================================== Files 305 305 Lines 22677 22674 -3 ========================================== - Hits 21553 21550 -3 Misses 1124 1124 ```

:umbrella: View full report in Codecov by Sentry.
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[TruongNhanNguyen]

Can you have a look on this PR, @vil02?

[TruongNhanNguyen]

Can you review this PR, @vil02?

[vil02]

* Rewrite `NQueensSolver` in OOP style

By the way: there is not much of OOP in this implementation (which is nothing bad). It is class based at most.

[TruongNhanNguyen]

* Rewrite `NQueensSolver` in OOP style

By the way: there is not much of OOP in this implementation (which is nothing bad). It is class based at most.

Thanks for pointing this out. I overused the OOP word in this context.