preliminary refactor for wildcard reborrows

[royAmmerschuber]

This is part of #4707 and only contains the refactoring & moving around of code. Functionality should be exactly the same as before.

[rustbot]

Thank you for contributing to Miri! A reviewer will take a look at your PR, typically within a week or two.
Please remember to not force-push to the PR branch except when you need to rebase due to a conflict or when the reviewer asks you for it.

[RalfJung]

This looks great, thanks! Please squash the commits. You can squash manually if there are multiple independent commits you want to preserve, or use ./miri squash (make sure to pick a suitable commit message). Then write @rustbot ready after you force-pushed the squashed PR.

@rustbot author

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[royAmmerschuber]

@rustbot ready