Save results to file instead of checking puzzles while solving

sud.h

@@ -47,6 +47,11 @@ enum check_res { INCOMPLETE, INCORRECT, SOLVED };
  */
 bool load(struct sudoku *sud, const char *ptr);
 
+/**
+ * Write the sudoku to the given pointer (`NCELLS` bytes).
+ */
+void save(struct sudoku *sud, char *ptr);
+
 /**
  * Attempt to update the cell at row `r`, column `c` to have the value
  * `val`. Returns `OK` on success, `ALREADY_DET` if the cell is