task: add an idle queue

Add a second level queue which is only serviced when the local and global
queues are empty.

At some point there might be a need to implement a full priority queue,
but for now two classes are sufficient.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

include/crucible/task.h

@@ -40,6 +40,9 @@ namespace crucible {
 		/// after the current instance exits.
 		void run() const;
 
+		/// Schedule task to run when no other Task is available.
+		void idle() const;
+
 		/// Schedule Task to run after this Task has run or
 		/// been destroyed.
 		void append(const Task &task) const;