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:

• I ran bellow commands using the latest version of rust nightly.
• I ran cargo clippy --all -- -D warnings just before my last commit and fixed any issue that was found.
• I ran cargo fmt just before my last commit.
• I ran cargo test just before my last commit and all tests passed.
• I added my algorithm to the corresponding mod.rs file within its own folder, and in any parent folder(s).
• I added my algorithm to DIRECTORY.md with the correct link.
• I checked COUNTRIBUTING.md and my code follows its guidelines.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@            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     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.