Add a "new_channel" for root fibers.

When new fibers are scheduled on the event loop, this new_channel
receives the newly created fibers. This lets a fiber track which fibers
have been added and let's a user implement a supervisor.

Fix formatting.

src/include/janet.h

@@ -842,7 +842,8 @@ struct JanetFiber {
 #ifdef JANET_EV
     JanetListenerState *waiting;
     uint32_t sched_id; /* Increment everytime fiber is scheduled by event loop */
-    void *supervisor_channel; /* Channel to push self to when signaling. */
+    void *done_channel; /* Channel to push self to when complete */
+    void *new_channel; /* Channel to push spawned children */
 #endif
 };