Support any type of flow id

Now, any value can be a flow id, not just a keyword.
The tradeoff is that we can't pass flows into some of our helper
functions, because a flow is a map and a map could simply be an
id.

src/re_frame/flow/alpha.cljc

@@ -15,10 +15,7 @@
 
 (def flows (r/atom {}))
 
-(defn lookup [x]
-  (cond
-    (flow? x) x
-    (id? x) (get @flows x)))
+(defn lookup [id] (get @flows id))
 
 (defn id [x]
   (cond-> x (flow? x) :id))