fixup! Add Iterator#cons returning a pair

src/iterator.cr

@@ -321,12 +321,12 @@ module Iterator(T)
   # Chunks of more than two items can be iterated using
   # `#cons(count : Int, reuse = false)`. This method is just an optimized
   # implementation for the special case of `size == 2` to avoid heap allocations.
-  def cons
+  def cons : Iterator({T, T})
     ConsTuple(typeof(self), T).new(self)
   end
 
   private struct ConsTuple(I, T)
-    include Iterator(Tuple(T, T))
+    include Iterator({T, T})
     include IteratorWrapper
 
     @last_elem : T | Iterator::Stop = Iterator::Stop::INSTANCE