Added a task wakeup callback. Closes #599.

The callback happens when a task moves from the "blocked" state to the
"running" state. The callback is also inherited by child tasks. There
is currently only a native API.

This code hasn't been heavily exercised yet.

src/rt/rust_scheduler.cpp

@@ -291,8 +291,10 @@ rust_scheduler::create_task(rust_task *spawner, const char *name) {
         new (this->kernel) rust_task (this, &newborn_tasks, spawner, name);
     DLOG(this, task, "created task: " PTR ", spawner: %s, name: %s",
                         task, spawner ? spawner->name : "null", name);
-    if(spawner)
+    if(spawner) {
         task->pin(spawner->pinned_on);
+        task->on_wakeup(spawner->_on_wakeup);
+    }
     newborn_tasks.append(task);
     return task;
 }