Make PhantomData dispatchable to write static methods on dyn-traits

[steffahn]

A TypedMetadata<T>-style struct (see link below for what I mean by that) however can technically be made to support this (which is something I've mentioned in that thread too):

Bikeshed request: `struct(<T as Pointee>::Metadata)`

TL;DR: // std::ptr struct <T: ?Sized>(pub <T as Pointee>::Metadata); impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<<U>> for <T> {} What should actually be called? (How is it different from <T as Pointee>::Metadata? The T type is reattached to the "strong type alias" / newtype.) Suggestions so far, unordered: JustMetadata<T> TypedMetadata<T> MetadataOf<T> (or some other predicate) ptr::Metadata<T> PointeeMetadata<T>