Add initial implementation for supervisor channels.

Supervisor channels are a simple concept to more efficiently
enable dynamic, structure concurrency. When a top-level fiber
completes (or errors), it will push itself to it's supervisor
channel if it has one (instead of printing a stacktrace). This
let's another fiber poll a channel and "supervise" a set of fibers.

src/include/janet.h

@@ -842,6 +842,7 @@ 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. */
 #endif
 };