Implement exhaustive search as script in main
DELETE exhaustive search related code

The method is too slow in solving sudoku: 
- sudoku_is_valid(sudoku_values) takes on average 11.7us to run.
- if using exhaustive search, the total time needed to solve the given
sudoku would be on the order of 9^26^11.7us = 2.4e12 years. 

Infeasible!