When referring to an associated type of a super trait, we used the substs of the
subtrait. That led to the #4931 crash if the subtrait had less parameters, but
it could also lead to other incorrectness if just the order was different.
Fixes#4931.
So e.g. if we have `fn foo<T: SomeTrait<u32>>() -> T::Item`, we want to lower
that to `<T as SomeTrait<u32>>::Item` and not `<T as SomeTrait<_>>::Item`.
The big change here is counting binders, not
variables (https://github.com/rust-lang/chalk/pull/360). We have to adapt to the
same scheme for our `Ty::Bound`. It's mostly fine though, even makes some things
more clear.
It improves compile time in `--release` mode quite a bit, it doesn't
really slow things down and, conceptually, it seems closer to what we
want the physical architecture to look like (we don't want to
monomorphise EVERYTHING in a single leaf crate).
