Merge #5211

Merge #5211

5211: Fix inference of indexing argument (partly) r=flodiebold a=flodiebold

We need to add the `T: Index<Arg>` obligation to be resolved later as well, otherwise we can't make inferences about `Arg` later based on the `Index` impls.

This still doesn't fix indexing with integer variables though; there's a further problem with Chalk floundering because of the variable, I think.

Co-authored-by: Florian Diebold <florian.diebold@freiheit.com>