$throw is a new kind of CPS term

* module/language/cps.scm ($throw): New term type that doesn't have a
  continuation.  Adapt all callers.  Remove now-unneeded
  "prune-bailouts" pass.

module/language/cps/effects-analysis.scm

@@ -614,6 +614,9 @@ the LABELS that are clobbered by the effects of LABEL."
         ;; what nonlocal predecessors of the handler do, so we
         ;; conservatively assume &all-effects.
         &all-effects)
+       (($ $kargs names syms ($ $throw))
+        ;; A reachable "throw" term can never be elided.
+        &all-effects)
        (($ $kreceive arity kargs)
         (match arity
           (($ $arity _ () #f () #f) &type-check)