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/typed-racket-pkgs/typed-racket-lib/typed-racket/types/abbrev.rkt

@@ -8,6 +8,7 @@
          racket/list
          racket/match
          racket/function
+         racket/undefined
          unstable/function
 
          (prefix-in c: (contract-req))
@@ -91,8 +92,8 @@
 (define/decl -Boolean (Un -False -True))
 (define/decl -Undefined
   (make-Base 'Undefined
-             #'(lambda (x) (equal? (letrec ([y y]) y) x)) ; initial value of letrec bindings
-             (lambda (x) (equal? (letrec ([y y]) y) x))))
+             #'undefined? ; initial value of letrec bindings
+             undefined?))
 (define/decl -Bytes (make-Base 'Bytes #'bytes? bytes?))
 (define/decl -Base-Regexp (make-Base 'Base-Regexp
                            #'(and/c regexp? (not/c pregexp?))

pkgs/typed-racket-pkgs/typed-racket-lib/typed-racket/utils/any-wrap.rkt

@@ -3,15 +3,14 @@
 (require racket/match racket/contract/combinator
          racket/fixnum racket/flonum
          racket/set
+         racket/undefined
          (only-in (combine-in racket/private/promise)
                   promise?
                   prop:force promise-forcer))
 
-(define undef (letrec ([x x]) x))
-
 (define (base-val? e)
   (or (number? e) (string? e) (char? e) (symbol? e)
-      (null? e) (regexp? e) (eq? undef e) (path? e)
+      (null? e) (regexp? e) (undefined? e) (path? e)
       (regexp? e) (keyword? e) (bytes? e) (boolean? e) (void? e)
       ;; Base values because you can only store flonums/fixnums in these
       ;; and not any higher-order values. This isn't sound if we ever