<prompt> body and handler are lambdas; add escape-only? field

* module/language/tree-il.scm (<prompt>): Change to have the body and
  handler be lambdas, and add an "escape-only?" field.  This will make
  generic prompts work better in CPS or ANF with the RTL VM, as it
  doesn't make sense in that context to capture only part of a frame.
  Escape-only prompts can still be fully inlined.
  (parse-tree-il, unparse-tree-il): Add escape-only? to the
  serialization.
  (make-tree-il-folder, pre-post-order): Deal with escape-only?.

* module/language/tree-il/analyze.scm (analyze-lexicals): Handle
  escape-only?, and the new expectations for the body and handler.

* module/language/tree-il/canonicalize.scm (canonicalize): Ensure that
  the body of an escape-only continuation is a thunk, and that the
  handler is always a lambda.
* module/language/tree-il/debug.scm (verify-tree-il): Assert that
  escape-only? is a boolean.

* module/language/tree-il/cse.scm (cse):
* module/language/tree-il/effects.scm (make-effects-analyzer):
* module/language/tree-il/peval.scm (peval):
* module/language/tree-il/primitives.scm (*primitive-expand-table*):
* test-suite/tests/peval.test ("partial evaluation"):
* module/language/tree-il/compile-glil.scm (flatten-lambda-case): Adapt
  to <prompt> change.

test-suite/tests/peval.test

@@ -1135,25 +1135,29 @@
    (call-with-prompt tag
                      (lambda () 1)
                      (lambda (k x) x))
-   (prompt (toplevel tag)
-           (const 1)
-           (lambda-case
-            (((k x) #f #f #f () (_ _))
-             (lexical x _)))))
+   (prompt #t
+           (toplevel tag)
+           (lambda _
+             (lambda-case
+              ((() #f #f #f () ())
+               (const 1))))
+           (lambda _
+             (lambda-case
+              (((k x) #f #f #f () (_ _))
+               (lexical x _))))))
 
   ;; Handler toplevel not inlined
   (pass-if-peval
-   (call-with-prompt tag
-                     (lambda () 1)
-                     handler)
-   (let (handler) (_) ((toplevel handler))
-        (prompt (toplevel tag)
-                (const 1)
-                (lambda-case
-                 ((() #f args #f () (_))
-                  (primcall apply
-                            (lexical handler _)
-                            (lexical args _)))))))
+      (call-with-prompt tag
+                        (lambda () 1)
+                        handler)
+    (prompt #f
+            (toplevel tag)
+            (lambda _
+              (lambda-case
+               ((() #f #f #f () ())
+                (const 1))))
+            (toplevel handler)))
 
   (pass-if-peval
    ;; `while' without `break' or `continue' has no prompts and gets its