One issue I see is that this adds another API (Scheduler) for people to have to
remember. They will have to know how/when to use Mutex, Thread, Fibers, perhaps
Guilds, and now Scheduler.
Is this really what we want to have in ruby? Aspects such as class Hash, String,
Array etc... are quite simple to use and understand. The whole parallelism part,
on the other hand, seems to spawn more and more complexity on its own.