all necessary changes to check references to uninitialized letrec variables

includes a new pass, letrec_check, two new primitives, and
changes to packages that grabbed the letrec undefined value

original commit: 72c958df628690ebc52a626d35cd4edcab0c7089

pkgs/compatibility-pkgs/compatibility-lib/mzlib/unit200.rkt

@@ -7,14 +7,13 @@
 		      syntax/name
 		      syntax/context
                       racket/syntax
+                      racket/undefined
 		      (only racket/base filter)
 		      "private/unitidmap.rkt")
 
   ;; ----------------------------------------------------------------------
   ;; Structures and helpers
 
-  (define undefined (letrec ([x x]) x)) ; initial value
-
   (define insp (current-inspector)) ; for named structures
 
   (define-struct unit (num-imports exports go)) ; unit value