Add load() for loading sudokus from text

sud.h

@@ -23,6 +23,7 @@
 
 #define SEGLEN 3
 #define NDIGITS (SEGLEN * SEGLEN)
+#define NCELLS (NDIGITS * NDIGITS)
 
 struct cellstate {
 	bool det;
@@ -40,6 +41,12 @@ enum update_res { NOT_ALLOWED, ALREADY_DET, OK };
 
 enum check_res { INCOMPLETE, INCORRECT, SOLVED };
 
+/**
+ * Read `NCELLS` values from the given pointer and load them into the
+ * sudoku.
+ */
+bool load(struct sudoku *sud, const char *ptr);
+
 /**
  * Populate the sudoku with some random values. The proportion of
  * cells that are filled will be approximately `fill_prop`.