This looks great, it's just a little bit subtle and I don't 100% understand yet. How did the ManuallyDrop logic prevent the queue reference count from decrementing, and how does the Option fix that?

@dherman Since Drop is recursive, ManuallyDrop prevents the fields from executing their Drop implementation as well. To drop the Arc would require some unsafe.

This changes the strategy so ManuallyDrop is no longer used. Instead, a None signals to the Drop implementation that nothing needs to be done. Since these fields are only used internally, the value will always be Some when a user interacts with it. The methods that take the value and replace it with None consume the struct. Since we control it, we can guarantee that only Drop will execute after the take.