networks: Make network? more precise.

Require that the hash table maps variables to procedures, instead of
requiring nothing of the values of the hash table.

networks.rkt

@@ -100,7 +100,7 @@
 (define update-function/c (-> state? any/c))
 
 ;;; A network is a mapping from its variables to its update functions.
-(define network? variable-mapping?)
+(define network? (hash/c variable? procedure?))
 
 ;;; Given a state s updates all the variables from xs.  This
 ;;; corresponds to a parallel mode.