* 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.
