Task: convert print_fn to a string

Since we are now unconditionally rendering the print_fn as a static
string, there is no need for it to be a function.  We also need it to
be brief and mostly constant.

Use a string instead.  Put the string before the function in the Task
constructor arguments so that the title string appears as a heading in
code, since we are making a breaking API change already.

Drop TASK_MACRO as it is broken by this change, but there is no similar
usage of Task anywhere to make it worth fixing.

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

src/bees.cc

@@ -151,9 +151,7 @@ BeesNote::get_name()
 	// about it being destroyed under us.
         auto current_task = Task::current_task();
         if (current_task) {
-                ostringstream oss;
-                oss << current_task;
-                return oss.str();
+		return current_task.title();
         }
 
         // OK try the pthread name next.