Attempt to make FocusMut::pair compatible with stacked borrows

[jplatte]

I wanted to take a stab at fixing #59 by making FocusMut::pair compatible with Stacked Borrows. I quickly ran into the limits of my understanding of this code though. Before I try harder, I wanted to ask if this is at all considered a useful change. We'd probably have to do the same thing for the triplet function right below pair for this to be useful.

[jneem]

I think this is might get complicated, especially with triplet. The issue is that the other representation is basically a tree of chunks, and if I understood this comment correctly, the only way to do this under stacked borrows is to retrieve both pointers on the same tree traversal. So then you'll have to repeat the logic in Vector::get and Node::index multiple times...

I'm not sure I understood everything completely, but I wonder if this can be solved by just keeping the leaves stored behind a *mut [T] or something. That way you can access that chunk of memory without going through any &mut reference to it, which might stop the stacked borrows from invalidating it?

[jplatte]

Yeah, that would be the other possibility (using NonNull though, to keep niche optimizations) but I think it would potentially push a lot of code into unsafe? Or maybe it's not actually that much..

Anyways, I'll close this since I think you're right that it requires a bunch of duplication.