BREAKING: Make domains part of the network.

I always keep bringing domains about for my network, which complicates
things a lot.  This commit packages the domains within the network,
which will break a lot of stuff :-) The following commits will fix it all.

networks.rkt

@@ -14,10 +14,12 @@
          graph racket/random racket/hash)
 
 (provide
- ;; Structures
  (struct-out dynamics)
+ ;; Structures
  (contract-out [struct tbf/state ([weights (hash/c variable? number?)]
-                                  [threshold number?])])
+                                  [threshold number?])]
+               [struct network ([functions (hash/c variable? procedure?)]
+                                [domains domain-mapping/c])])
  ;; Functions
  (contract-out [update (-> network? state? (set/c variable? #:kind 'dont-care) state?)]
                [make-state (-> (listof (cons/c symbol? any/c)) state?)]
@@ -166,8 +168,9 @@
 ;;; state.
 (define update-function/c (-> state? any/c))
 
-;;; A network is a mapping from its variables to its update functions.
-(define network? (hash/c variable? procedure?))
+;;; A network is a mapping from its variables to its update functions,
+;;; together with the domains of the variables.
+(struct network (functions domains))
 
 ;;; Given a state s updates all the variables from xs.  This
 ;;; corresponds to a parallel mode.