[iter] Add unary operator+ that returns a copy

src/hb-iter.hh

@@ -69,6 +69,7 @@ struct hb_iter_t
 
   /* Operators. */
   iter_t iter () const { return *thiz(); }
+  iter_t operator + () const { return *thiz(); }
   explicit_operator bool () const { return thiz()->__more__ (); }
   unsigned len () const { return thiz()->__len__ (); }
   hb_remove_reference (item_t)* operator -> () const { return hb_addressof (**thiz()); }

src/test-iter.cc

@@ -135,7 +135,7 @@ main (int argc, char **argv)
   hb_array_t<hb_vector_t<int> > pa;
   pa->as_array ();
 
-  s
+  + s
   | hb_map (hb_identity)
   | hb_filter ()
   | hb_filter (hb_bool)