Add "pop" field to $prompt

Add "pop" field to $prompt

* module/language/cps.scm ($prompt): Add a "pop" field, indicating the
  continuation at which this prompt is popped.  The body of the prompt
  is dominated by the prompt, and post-dominated by the pop.  Adapt all
  builders and users.

* module/language/cps/closure-conversion.scm:
* module/language/cps/compile-rtl.scm:
* module/language/cps/slot-allocation.scm:
* module/language/cps/verify.scm:
* module/language/tree-il/compile-cps.scm: Adapt.

* module/language/cps/dfg.scm (visit-fun): Add an arc from the pop to
  the handler, to keep handler variables alive through the prompt body.