effects macro: Add vis where needed

[teohhanhui]

This (properly) allows effects to live in a separate module from the handlers, while using the effects! macro.

TODO:

• [x] How do we handle tuple struct field visibility to allow pattern matching? If we allow visibility modifiers it sort of breaks the illusion of it being a function signature lol

[teohhanhui]

How do we handle tuple struct field visibility to allow pattern matching? If we allow visibility modifiers it sort of breaks the illusion of it being a function signature lol

~~Should we just remove the fn keyword instead?~~ No.

[teohhanhui]

/ping @rosefromthedead :see_no_evil:

[rosefromthedead]

Thanks! There's a bit of churn in here for a small change (namely, maybe_phantom_data has become group_phantom_data_field and group_phantom_data has been introduced, neither of which seem necessary) - please keep these unrelated changes to a minimum or split them into a separate commit to make the body of the PR easier to review.

LGTM though :)