Currently all fiber threads end up with a name something like "Ruby-Fiber-15" because we pool those thread objects and reuse them. However, this does not provide very good information about where or why the fiber thread was enlisted.

We could setName on those threads to update the name to something that reflects the origins of the fiber or enumerator using the given fiber thread, which would make it easier to debug when there's a leak or some other issue with those coroutine threads.