twn_util.c: add file_read()

include/twn_util.h

@@ -42,6 +42,9 @@
 
 #endif /* TWN_NOT_C */
 
+/* read file to null terminated string, it is freed when the frame ends */
+TWN_API char const *file_read(char const *file);
+
 /* calculates the overlap of two rectangles */
 TWN_API Rect rect_overlap(Rect a, Rect b);
 /* returns true if two rectangles are intersecting */