Solves a Sudoku game using a depth-first search with some intermediate logic.
After loading the "search.lisp" file, boards should be constructed as 2D arrays
of numbers, with the symbol - representing an empty square.
A board may be solved by making a new game object and passing it to
solve-game as follows:
(setq game (make-instance 'game 'board *test9*))
(setq sol (solve-game game))