RFC for "fat objects" for DSTs

This is intended as an alternative to #5.
This line doesn't seem right to me.  You said up above that given `Fat<U,T>`, then `T` must be a concrete type, and `U` must be an existential variant of that type.  But here you have written `Fat<HTMLVideoElement,Element>`, and while  `HTMLVideoElement` itself has not been given a definition, we *know* that the occurrence of `Element` there cannot be right, since `Element` is a trait, not a concrete type.

I suspect that either:
 * you meant to write `Fat<Element,HTMLVideoElement>` (i.e. you just swapped the order of the arguments), and thus `HTMLVideoElement` is implicitly a concrete type, or
 * in keeping with the conventions you established above, you actually meant to write `Fat<HTMLVideoElement, HTMLVIdeoElementFields>`, and implicitly `HTMLVideoElement` is some trait that indirectly extends the `trait Element`.

It seems to me like the second bullet is the more likely one, but it would be good to clarify either way.