Remove constructor from hb_prealloced_array_t

This was causing all object types to be non-POD and have static
initializers.  We don't need that!

Now, most nil objects just moved from .bss to .data.  Fixing for that
coming soon.

src/hb-object-private.hh

@@ -80,6 +80,8 @@ struct hb_user_data_array_t
 
   hb_lockable_set_t<hb_user_data_item_t, hb_static_mutex_t> items;
 
+  inline void init (void) { items.init (); }
+
   HB_INTERNAL bool set (hb_user_data_key_t *key,
 			void *              data,
 			hb_destroy_func_t   destroy,
@@ -111,6 +113,7 @@ struct hb_object_header_t
 
   inline void init (void) {
     ref_count.init (1);
+    user_data.init ();
   }
 
   inline bool is_inert (void) const {

src/hb-private.hh

@@ -247,7 +247,7 @@ struct hb_prealloced_array_t {
   Type *array;
   Type static_array[StaticSize];
 
-  hb_prealloced_array_t (void) { memset (this, 0, sizeof (*this)); }
+  void init (void) { memset (this, 0, sizeof (*this)); }
 
   inline Type& operator [] (unsigned int i) { return array[i]; }
   inline const Type& operator [] (unsigned int i) const { return array[i]; }
@@ -351,6 +351,8 @@ struct hb_lockable_set_t
 {
   hb_array_t <item_t> items;
 
+  inline void init (void) { items.init (); }
+
   template <typename T>
   inline item_t *replace_or_insert (T v, lock_t &l, bool replace)
   {

src/hb-set-private.hh

@@ -37,6 +37,7 @@
 struct _hb_set_t
 {
   inline void init (void) {
+    header.init ();
     clear ();
   }
   inline void fini (void) {