Uses the `DuckTyping` module to approximate whether something is likely
to be an iterator or not.

We lose one test result due to the fact that we don't know what to do
about `for ... in 1` (because `1` is an instance of a built-in). I'm
going to defer addressing this until we get some modelling of built-in
types.

A common pattern is to check `isinstance(it, (list, tuple))` before
proceeding with the iteration.