Make ground async-channel always available, rather than dynamically

scoped to run-ground. This is important because (spawn-timer-driver),
for example, is called outside run-ground and spawns a thread that
communicates with ground via the ground async-channel. When we move to
running multiple grounds at once, we will have to revisit the
parameterization scheme here.

minimart/ground.rkt

@@ -21,14 +21,12 @@
 ;; (Parameterof (Option AsyncChannel))
 ;; Communication channel from auxiliary (usually driver) threads to
 ;; the currently-active ground VM.
-(define current-ground-event-async-channel (make-parameter #f))
+(define current-ground-event-async-channel (make-parameter (make-async-channel)))
 
 ;; Any -> Void
 ;; Sends a (non-feedback) message at the ground-VM metalevel.
 (define (send-ground-message body)
-  (match (current-ground-event-async-channel)
-    [(? async-channel? ch) (async-channel-put ch (send body))]
-    [_ (error 'send-ground-message "Called outside dynamic scope of run-ground")]))
+  (async-channel-put (current-ground-event-async-channel) (send body)))
 
 ;; RacketEvent -> RacketEvent
 ;; Wraps a CML-style Racket event with a handler that sends the event
@@ -61,7 +59,6 @@
 ;; Action* -> Void
 ;; Runs a ground VM, booting the outermost World with the given Actions.
 (define (run-ground . boot-actions)
-  (parameterize ((current-ground-event-async-channel (make-async-channel)))
   (let await-interrupt ((inert? #f) (p (spawn-world boot-actions)) (active-events '()))
     (define active-gestalt (process-gestalt p))
     (define event-list (if inert?
@@ -92,4 +89,4 @@
 		     (void)]
 		    [_
 		     (log-warning "run-ground: ignoring useless meta-action ~v" a)
-		       (process-actions actions g)])]))]))))))
+		     (process-actions actions g)])]))])))))