fixed let not coying the initial value nodes

bin/tests/lexical_scope.slime.expanded

@@ -0,0 +1,18 @@
+(define (make-counter) (let ((var 0.000000)) (lambda nil (mutate var (+ 1.000000 var)) var)))
+
+(define counter1 (make-counter))
+
+(assert (= (counter1) 1.000000))
+
+(define counter2 (make-counter))
+
+(assert (= (counter2) 1.000000))
+
+(assert (= (counter2) 2.000000))
+
+(assert (= (counter1) 2.000000))
+
+(assert (= (counter1) 3.000000))
+
+(assert (= (counter2) 3.000000))
+