network: Roll back update to its previous version.

I thought I was clever to rewrite update using for/hash, but in fact
this new version only included the updated variables in the new
state :D

networks.rkt

@@ -203,8 +203,10 @@
 ;;; Given a state s updates all the variables from xs.
 (define (update network s xs)
   (define funcs (network-functions network))
-  (for/hash ([x xs])
-    (values x ((hash-ref funcs x) s))))
+  (for/fold ([new-s s])
+            ([x xs])
+    (define fx (hash-ref funcs x))
+    (hash-set new-s x (fx s))))
 
 (module+ test
   (test-case "basic definitions"