[iter] Make hb_fill() and hb_copy() take iterators

I'm still going back and force...

src/test-iter.cc

@@ -44,19 +44,16 @@ struct array_iter_t : hb_iter_t<array_iter_t<T>, T>
   hb_array_t<T> arr;
 };
 
-template <typename C, typename V> inline void
-hb_fill (const C &c, const V &v)
+template <typename I, typename V> inline void
+hb_fill (hb_iter_t<I> i, const V &v)
 {
-  for (typename C::iter_t i = c.iter (); i; i++)
+  for (; i; i++)
     hb_assign (*i, v);
 }
 
 template <typename S, typename D> inline bool
-hb_copy (const D &d, const S &s)
+hb_copy (hb_iter_t<D> id, hb_iter_t<S> is)
 {
-  typename S::iter_t is = s.iter ();
-  typename D::iter_t id = d.iter ();
-
   for (; id && is; ++id, ++is)
     *id = *is;