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
(sorry late response)

The problem is that `Stx-Listof` is really a type constructor, and TR can't currently reconstruct the original type from a type constructor application. It would be nice if this were possible.