Update README to have latest command examples

Fine tune README and comments

Update to use one dimension slice to store values, and change

the square ID as integer 0~80 to replace original map structure.

First version to have benchmarks and test cases with known puzzles

- Hide those variables used inside the package to unexported
- Export only "Solve" and "Display" functions
- Add more test cases, example code, and benchmarks to test code
- Update the performance result of this first version to README