add void to end of module begin of stlc; add some recursive tests

stlc-tests.rkt

@@ -78,3 +78,7 @@
                        => (cons {Int} 2 (cons {Int} 3 (null {Int}))))
 (check-not-type (map add1 (cons {Int} 1 (cons {Int} 2 (null {Int})))) 
                 : (Listof String))
+
+(define (a [x : Int]) (b x))
+(define (b [x : Int]) (a x))
+(define (ff [x : Int]) (ff x))

stlc.rkt

@@ -336,7 +336,7 @@
 ;     #:when (printf "mb: ~a\n" (syntax->datum (expand/df #'(letrec ([f v] ...) e ...))))
      (quasisyntax/loc stx 
        (#%module-begin
-        #,(expand/df #'(letrec/tc ([f : τ v] ...) e ...))
+        #,(expand/df #'(letrec/tc ([f : τ v] ...) e ... (void)))
         (define #,(datum->syntax stx 'runtime-env)
           (for/hash ([x:τ '#,(map (λ (xτ) (cons (car xτ) (syntax->datum (cdr xτ))))
                                   (hash->list (do-subst (Γ))))])