Refactor NQueens implementation (#732)

* ref: refactor NQueens implementation
- 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

* ref: refactor implementation
- Simplified the `NQueensSolver` struct initialization
- Streamlined the `is_safe` method
- Minor formatting changes

* ref: update implementation
- Use `std::mem::take()` instead of `clone`
- Rewrite tests

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Co-authored-by: Piotr Idzik <65706193+vil02@users.noreply.github.com>