fix types of stx->list, stx-car, etc.

see also https://github.com/plt/racket/commit/ada4fb74fa28b0f72ed590785afb9ebe08dded45 and https://github.com/plt/racket/commit/99c8da4d92259251c6fbdac38969c16dec80ef62
Is there any way to connect things like `-Stx-Listof` and `Stx-Listof` to each other so that the type-checker knows they're the same thing?  
Because right now, the printed types for `stx->list` etc. have `(Rec lst (U Stx-Null (Pairof a lst) (Syntaxof (Pairof a lst))))` instead of `(Stx-Listof a)`.  Is there any way to fix that?