support higher-ranked types for chalk

[nikomatsakis]

As discussed in the WG-traits planning meeting today, we need to decide how we will represent higher-ranked types in rustc. We decided to try for using debruijn indexing more universally, much as it is done in chalk. The plan is to generalize the existing CanonicalVar so that it represents a "bound" thing -- probably it just gets unified with DebruijnIndex. This will take a few steps, each of which I plan to describe in its own sub-issue:

• Cleanup Binder fields (#49814)
• Make DebruijnIndex 0-based (#49813)
• Unify CanonicalVar and DebruijnIndex (#49887)
• Maybe cleanup how regions are handled to use distinct indices for each region, rather than names (somewhat separate)

[nikomatsakis]

cc @rust-lang/wg-traits

[scalexm]

I believe this can be closed, in light of #55330, #55649 and #55921.