9dd3288557
Make `ThinBox<T>` covariant in `T` Just like `Box<T>`, we want `ThinBox<T>` to be covariant in `T`, but the projection in `WithHeader<<T as Pointee>::Metadata>` was making it invariant. This is now hidden as `WithOpaqueHeader`, which we type-cast whenever the real `WithHeader<H>` type is needed. Fixes the problem noted in <https://github.com/rust-lang/rust/issues/92791#issuecomment-1104636249>.