fix: consider outer binders when folding captured items' type
Fixes#14966
Basically, the crash is caused by us producing a broken type and passing it to chalk: `&dyn for<type> [for<> Implemented(^1.0: A<^0.0>)]` (notice the innermost bound var `^0.0` has no corresponding binder). It's created in `CapturedItemWithoutTy::with_ty()`, which didn't consider outer binders when folding types to replace placeholders with bound variables.
The fix is one-liner, but I've also refactored the surrounding code a little.
2f1b7cedcf