This makes it easier to work with lazy iterators used for callables,
etc. when extracting items or comparing to lists, e.g. during testing.
Also added `BaseChoiceIterator.__iter__()` to make it clear that
subclasses must implement this and added `__all__` to the module.
Co-authored-by: Adam Johnson <me@adamj.eu>
Co-authored-by: Natalia Bidart <124304+nessita@users.noreply.github.com>