The solver can only solve when at least one cell can be solved with certainty
at each step of the process until repeating this eventually results in a solved
board.
The next step here is to allow exploration/trail of possible solutions.
Presumably, would want to keep a copy of the board just before each "guess"
that's taken to be able to easily roll back if failure.

This would probably work really nicely as a recursive call:

	board = get_fresh_board(board)
	solve_all_certainties(board)
	for each possible board as cand_board:
		solution = solve_all_certainties(cand_board)
		if complete(solution)
			success
	/* we should have solution presuming board was solvable */