Add Fiber.suspend

[ysbaddaden]

Replaces the private Crystal::Scheduler.reschedule method that suspends the execution of the current fiber (that must be manually re-enqueued to be resumed) with a public method: Fiber.suspend.

Note

It could be Fiber.reschedule. I'm not sure which term is more explicit and closer to the intent?

Related to the other PRs that abstract Crystal::Scheduler away.

Relates to: #5204

[straight-shoota]

I think suspend fits better than reschedule. The latter works in the context of Scheduler, but we're not rescheduling the fiber, we're suspending it.