Update TaggedStructure::push lifetimes

[SteveIntensifies]

Right now, TaggedStructure::push is defined by #994 as:

fn push<T: Extends<Self> + TaggedStructure<'a>>(mut self, next: &'a mut T) -> Self

This requires that the extending structure has the same lifetime as the base structure, which is unnecessarily restricting.

This PR updates the function signatures of push and extend to require that the extending structure outlives the base structure.

[Ralith]

I can't think of any reason why this would be wrong, though I'm not super great at reasoning about variance.

[MarijnS95]

This manually-reimplemented lifetime/borrow-checking surrounding a bunch of raw pointers is tricky and finicky to get right, that's exactly why we have a bunch of tests around it to ensure expected correctness as well as usability.

As part of this change I'd appreciate to see an additional test that demonstrates what currently isn't supported (I can vaguely assume), and so that this doesn't regress in the future.

Also, welcome back ;)

[SteveIntensifies]

That's a fair request. You don't need an additional test because the existing test would cover this case with minor modifications. I just had to make variable_pointers and corner to have different scopes.

Also, welcome back ;)

I'm totally new here 😅