[vector] Introduce realloc_moves tag

Some types (even aggregate) can be moved using realloc().
Extend the fast path to hb_hashmap and tuple_delta_t.

src/hb-array.hh

@@ -47,6 +47,8 @@ enum hb_not_found_t
 template <typename Type>
 struct hb_array_t : hb_iter_with_fallback_t<hb_array_t<Type>, Type&>
 {
+  static constexpr bool realloc_move = true;
+
   /*
    * Constructors.
    */

src/hb-map.hh

@@ -42,6 +42,8 @@ template <typename K, typename V,
 	  bool minus_one = false>
 struct hb_hashmap_t
 {
+  static constexpr bool realloc_move = true;
+
   hb_hashmap_t ()  { init (); }
   ~hb_hashmap_t () { fini (); }
 

src/hb-ot-var-common.hh

@@ -434,6 +434,8 @@ enum packed_delta_flag_t
 
 struct tuple_delta_t
 {
+  static constexpr bool realloc_move = true;  // Watch out when adding new members!
+
   public:
   hb_hashmap_t<hb_tag_t, Triple> axis_tuples;
 

src/hb-vector.hh

@@ -37,6 +37,8 @@ template <typename Type,
 	  bool sorted=false>
 struct hb_vector_t
 {
+  static constexpr bool realloc_move = true;
+
   typedef Type item_t;
   static constexpr unsigned item_size = hb_static_size (Type);
   using array_t = typename std::conditional<sorted, hb_sorted_array_t<Type>, hb_array_t<Type>>::type;
@@ -268,10 +270,9 @@ struct hb_vector_t
     }
     return new_array;
   }
-  /* Specialization for hb_vector_t<hb_{vector,array}_t<U>> to speed up. */
+  /* Specialization for types that can be moved using realloc(). */
   template <typename T = Type,
-	    hb_enable_if (hb_is_same (T, hb_vector_t<typename T::item_t>) ||
-			  hb_is_same (T, hb_array_t <typename T::item_t>))>
+	    hb_enable_if (T::realloc_move)>
   Type *
   realloc_vector (unsigned new_allocated, hb_priority<1>)
   {