Higher-ranked trait bounds

A description of higher-ranked trait bounds, which is the remaining piece needed to make unboxed closures equal to normal closures.

[Rendered view.](https://github.com/nikomatsakis/rfcs/blob/hrtb/active/0000-higher-ranked-trait-bounds.md)


@pnkfelix That's essentially https://github.com/rust-lang/rfcs/pull/122, which was postponed.