Support capturing multiple value type in `forall`, etc.

[evhan]

As requested by alaricsp in ​this thread, it would be nice if one could capture and refer to multiple-valued return types with forall, e.g.:

(: call-with-foo (forall (a) (foo (-> . a) -> . a)))

(define (call-with-foo foo thunk)
  ; do some stuff and return whatever thunk does,
  ; even if it's multi-valued:
  (thunk))

(The above is just a sketch, not necessarily the right way to express this..)