[map] Place item hash between key and value, not after them

This way if only one of key and value is 64bit (eg. pointer), and other is 32bit,
the whole item will fit in 128bit, whereas before it would have been bumped up to
196 if only value was 64bit (a common use-case for us.)

src/hb-map.hh

@@ -68,8 +68,8 @@ struct hb_hashmap_t
   struct item_t
   {
     K key;
-    V value;
     uint32_t hash;
+    V value;
 
     void clear ()
     {