Add initial implementation of user_data to objects

src/hb-common.h

diff --git a/src/hb-common.h b/src/hb-common.h
index e6f5f2b..74030f4 100644
--- a/src/hb-common.h
+++ b/src/hb-common.h
@@ -68,8 +68,6 @@
   int8_t i8[4];
 } hb_var_int_t;
 
-typedef void (*hb_destroy_func_t) (void *user_data);
-
 
 /* hb_tag_t */
 
@@ -287,6 +285,15 @@
 hb_script_get_horizontal_direction (hb_script_t script);
 
 
+/* User data */
+
+typedef struct _hb_user_data_key_t {
+  char unused;
+} hb_user_data_key_t;
+
+typedef void (*hb_destroy_func_t) (void *user_data);
+
+
 HB_END_DECLS
 
 #endif /* HB_COMMON_H */