Refactor Sudoku implementation

[TruongNhanNguyen]

Description

• Create sudoku_solver function to hide states of SudokuSolver class
• Rename variables to make code more readable
• Rewrite tests using macros
• Remove print_board method.

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 94.90%. Comparing base (1020ea2) to head (e1b9278). Report is 7 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #715 +/- ## ========================================== + Coverage 94.77% 94.90% +0.12% ========================================== Files 298 301 +3 Lines 22194 22344 +150 ========================================== + Hits 21035 21206 +171 + Misses 1159 1138 -21 ```

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