Use `dataflow::ResultsCursor` in borrowck

[matthewjasper]

Having a ResultsCursor stored in MirBorrowckCtxt would allow accessing the flow state at arbitrary points in error reporting and make using the usual mir visitor possible.

[pierwill]

Is there work on this that you'd like to see done on this, @matthewjasper? I'd like to work on something MIR-related, but would need some guidance.

[matthewjasper]

I don't really have time to mentor this, but the linked pr had a comment that provides some context. That said, this is not really a great starting point for work on MIR since it's a refactoring to improve some errors in edge cases.

I'm not really sure what the best place to start with for MIR is, perhaps writing a MIR optimisation or looking for a more easily fixed borrow check error quality problem (but I'm not sure how many of those there are left).