[array] Add chop()

Takes n, and returns iterator of iterators that contain up to
n items each.  Basically cuts the array into subarrays of size n.
The last sub-array might contain fewer.

Ideally this should become a generic iter tool, not array-specific,
so we can use it in GPOS to chop a value matrix into rows and elements.

src/hb-array.hh

@@ -183,6 +183,20 @@ struct hb_array_t : hb_iter_with_fallback_t<hb_array_t<Type>, Type&>
 
   hb_array_t truncate (unsigned length) const { return sub_array (0, length); }
 
+  auto chop (unsigned n) const -> decltype
+  (
+    + hb_range (0u, 0u, n)
+    | hb_map (hb_partial<1> (hb_add, *this))
+    | hb_map (hb_partial<2> (&hb_array_t::truncate, n))
+  )
+  {
+    return
+    + hb_range (0u, length, n)
+    | hb_map (hb_partial<1> (hb_add, *this))
+    | hb_map (hb_partial<2> (&hb_array_t::truncate, n))
+    ;
+  }
+
   template <typename T,
 	    unsigned P = sizeof (Type),
 	    hb_enable_if (P == 1)>