fix event-bus:subscribe

seems to be a Fennel bug:
<andreyorst> the problem is that tset is taken out of the or and nil is placed in place of it, thus making side effect occur
<andreyorst> even though it shouldn't
<andreyorst> telent: good catch

just/just.fnl

@@ -21,7 +21,7 @@
 (fn event-bus []
   (let [subscriptions {}
         vivify (fn [n v]
-                 (or (. n v) (tset n v {}))
+                 (tset n v (or (. n v) []))
                  (. n v))]
     {
      :subscriptions subscriptions