iterator: use iota for izip

Using `indices` introduced problematic dangling references. We should
almost always want integral indices anyway, so we drop the fancy
iterator. A user can always call zip with an indices object anyway.

iterator.hpp

@@ -515,7 +515,7 @@ namespace cruft {
     ///
     /// eg, cruft::zip ({1,2,3}, {4,5,6}) ~= {{1,4},{2,5},{3,6}}
     template <typename ...ContainerT>
-    auto
+    decltype(auto)
     zip (ContainerT&&... data)
     {
         CHECK (((std::size (data) == std::size (variadic::get<0> (data...))) && ...));
@@ -531,15 +531,21 @@ namespace cruft {
     /// object where the first of the iterator's value_types is the index of
     /// that iterator. ie, It combines container offsets with value_types.
     ///
+    /// The supplied containers _must_ not change size while the izip object
+    /// is live. The size of the containers may be cached for efficiency.
+    ///
     /// eg, cruft::izip ("abc") ~= {{0,'a'},{1,'b'},{2,'c'}}
     template <typename ...ContainerT>
-    auto
+    decltype(auto)
     izip (ContainerT&&... data)
     {
-        indices idx (::cruft::variadic::get<0> (data...));
-
+        // Assume that all containers are the same size and create a range
+        // that will supply integral indices over the first container.
+        //
+        // The call to `zip` will peform more rigorous tests on the sizes
+        // of the container collection.
         return zip (
-            std::move (idx),
+            iota (::cruft::variadic::get<0> (data...).size ()),
             std::forward<ContainerT> (data)...
         );
     }