refactor example for solving a Sudoku, add second strategy for solving.
refactor example for solving a Sudoku, add second strategy for solving.

The existing strategy is based on 9*9=81 integer variables in range 1-9.
The new strategy is purely SAT-based on 9*9*9=729 boolean variables.
The new strategy is several times faster than the existing one.

sosy-lab/java-smt,sosy-lab/java-smt,sosy-lab/java-smt,sosy-lab/java-smt,sosy-lab/java-smt