higher-ranked where-clauses, outlives

[nikomatsakis]

We don't have any tests for Rust features like

fn foo<T>()
where
    for<'a> T: Trait<'a>
{}

we should fix that (and fix the borrow checker to support it!)

[BoxyUwU]

figuring out how to model implied bounds on binders in formality is something I'm quite interested in as it's related to a lot of our open soundness bugs and also some of the work im currently doing 🤔 I imagine this issue is just for basic support though, rather than figuring out a perfect story for the eventually correct handling of higher ranked stuff