For now, we are focusing only on iterating over the argument ids and not
the values.

This provides a building block for more obscure use cases like iterating
over argument values, in order.  We are not providing it out of the box
at the moment both to not overly incentize a less common case, because
it would abstract away a performance hit, and because we want to let
people experiment with this and if a common path emerges we can consider
it then if there is enough users.

Fixes clap-rs#1206