fixed let not coying the initial value nodes

bin/tests/lexical_scope.slime

@@ -0,0 +1,16 @@
+(define (make-counter)
+  (let ((var 0))
+    (lambda ()
+      (mutate var (+ 1 var))
+      var)))
+
+(define counter1 (make-counter))
+(assert (= (counter1) 1))
+
+(define counter2 (make-counter))
+(assert (= (counter2) 1))
+
+(assert (= (counter2) 2))
+(assert (= (counter1) 2))
+(assert (= (counter1) 3))
+(assert (= (counter2) 3))