Use iolists in collectable for binaries

lib/elixir/lib/collectable.ex

@@ -85,8 +85,8 @@ defimpl Collectable, for: BitString do
 
   def into(original) do
     {original, fn
-      bitstring, {:cont, x} -> <<bitstring :: bits, x :: bits>>
-      bitstring, :done -> bitstring
+      acc, {:cont, x} when is_bitstring(x) -> [acc|x]
+      acc, :done -> IO.iodata_to_binary(acc)
       _, :halt -> :ok
     end}
   end