Backport of e2bf2b5

The instance checks in the `iterator` module use `KValue::is_iterable`
to decide whether or not to accept the instance provided to the call.

When used as standalone functions, adaptors like
`iterator.enumerate(1..10)` were accepting the `iterator` module causing
an error to be thrown.

The fix is to extend the `is_iterable` logic to only accept object-like
maps if they implement `@iterator` or `@next`.