task: allow external access to Task print function

This enables bees' thread introspection to use task descriptions in
status and log messages.

BeesNote will be calling Task::current_task() from non-Task contexts,
which means we need to allow Task's shared state pointer to be null.
Remove some asserts that will ruin our day in that case.

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

include/crucible/task.h

@@ -32,13 +32,22 @@ namespace crucible {
 		// schedule Task before other queued tasks
 		void run_earlier() const;
 
+		// describe Task as text
+		ostream &print(ostream &os) const;
+
 		// Returns currently executing task if called from exec_fn.
 		// Usually used to reschedule the currently executing Task.
 		static Task current_task();
 
+		// Ordering for containers
 		bool operator<(const Task &that) const;
+
+		// Null test
+		operator bool() const;
 	};
 
+	ostream &operator<<(ostream &os, const Task &task);
+
 	class TaskMaster {
 	public:
 		// Blocks until the running thread count reaches this number