Refactor various "type testing" traits to share code

There are various traits (`RegionEscape`, `HasProjectionTypes`, `ReferencesError`) that are useful for checking properties on all the types/regions reachable through a value. Right now they each have their own separate impls for things like `FnSig` and `Rc<T>`. It'd be nice to use a setup like `TypeFoldable` instead to save code.
