optimize the reading of strings in the grammar loader.

src/runtime/c/gu/utf8.h

@@ -29,7 +29,12 @@ gu_out_utf8(GuUCS ucs, GuOut* out, GuExn* err)
 	}
 }
 
+// Helper functions used in other modules
+
 GuUCS
 gu_utf8_decode(const uint8_t** utf8);
 
+void
+gu_in_utf8_buf(uint8_t** buf, GuIn* in, GuExn* err);
+
 #endif // GU_UTF8_H_