Continuation labels and variable identifiers may be integers

* module/language/cps.scm (label-counter, var-counter): New parameters,
  for producing fresh label and var names.
  (fresh-label, fresh-var): New procedures.
  (let-fresh): New macro, will replace let-gensyms.
  (build-cps-term): Use let-fresh.

* module/language/tree-il/compile-cps.scm: Use let-fresh to generate
  fresh names.

* module/system/vm/assembler.scm (make-meta, begin-kw-arity): Allow
  exact integers as labels.
  (link-debug): Explicitly mark low-pc as being an "addr" value.

module/language/cps.scm

@@ -123,8 +123,12 @@
             ;; Expressions.
             $void $const $prim $fun $call $callk $primcall $values $prompt
 
+            ;; Fresh names.
+            label-counter var-counter
+            fresh-label fresh-var
+            let-fresh let-gensyms
+
             ;; Building macros.
-            let-gensyms
             build-cps-term build-cps-cont build-cps-exp
             rewrite-cps-term rewrite-cps-cont rewrite-cps-exp
 
@@ -187,6 +191,26 @@
 (define-cps-type $values args)
 (define-cps-type $prompt escape? tag handler)
 
+(define label-counter (make-parameter #f))
+(define var-counter (make-parameter #f))
+
+(define (fresh-label)
+  (let ((count (label-counter)))
+    (label-counter (1+ count))
+    count))
+
+;; FIXME: Currently vars and labels need to be unique, so we use the
+;; label counter.
+(define (fresh-var)
+  (let ((count (label-counter)))
+    (label-counter (1+ count))
+    count))
+
+(define-syntax-rule (let-fresh (label ...) (var ...) body ...)
+  (let ((label (fresh-label)) ...
+        (var (fresh-var)) ...)
+    body ...))
+
 (define-syntax let-gensyms
   (syntax-rules ()
     ((_ (sym ...) body body* ...)
@@ -261,7 +285,7 @@
     ((_ ($letconst () body))
      (build-cps-term body))
     ((_ ($letconst ((name sym val) tail ...) body))
-     (let-gensyms (kconst)
+     (let-fresh (kconst) ()
        (build-cps-term
          ($letk ((kconst ($kargs (name) (sym) ($letconst (tail ...) body))))
            ($continue kconst (let ((props (source-properties val)))