Add filled() function as a faster alternative to check()

sud.h

@@ -71,6 +71,11 @@ void print(const struct sudoku *sud);
  * Determine whether the sudoku has been solved correctly, contains
  * invalid choices or is incomplete.
  */
-enum check_res check(const struct sudoku *sud);    
+enum check_res check(const struct sudoku *sud);
+
+/**
+ * Determine whether all the sudoku's cells have been determined.
+ */
+bool filled(const struct sudoku *sud);
 
 #endif