Minor change to hb_language_t

Let me know if this breaks anyone's anything.

src/hb-common.cc

@@ -187,7 +187,7 @@ struct hb_language_item_t {
     return *this;
   }
 
-  void finish (void) { free (lang); }
+  void finish (void) { free ((void *) lang); }
 };
 
 

src/hb-common.h

@@ -131,7 +131,7 @@ hb_direction_to_string (hb_direction_t direction);
 
 /* hb_language_t */
 
-typedef struct hb_language_impl_t *hb_language_t;
+typedef const struct hb_language_impl_t *hb_language_t;
 
 /* len=-1 means str is NUL-terminated */
 hb_language_t