Since we're not using those functions as argparse callbacks anymore, we
can write a normal function instead of factories, which simplifies
things a lot.
This means:
- An annotation like (int, str) is now typing.Union[int, str].
- utils.typing got expanded so it acts like the real typing.py, with
issubclass() working properly with typing.Union and __union_params__
being set.
- A literal string doesn't exist anymore as annotation, instead
@cmdutils.argument now has a 'choices' argument which can be used like
@cmdutils.argument('arg', choices=['val1', 'val2']).
- Argument validating/converting is now entirely handled by
argparser.type_conv instead of relying on python's argparse, i.e.
type/choices is now not passed to argparse anymore.
