Abstract Crystal::Scheduler methods

[ysbaddaden]

Changes are internal (private), except for Fiber.suspend that introduces a new public method.

• Add Crystal::EventLoop.current instead of Crystal::Scheduler.event_loop
• Use Fiber#enqueue instead of Crystal::Scheduler.enqueue
• Add Fiber.suspend instead of Crystal::Scheduler.reschedule
  • alternative: it could be Fiber.reschedule

This is part 2 of abstracting Crystal::Scheduler away.

[straight-shoota]

I believe these commits should be individual PRs. Especially considering that some are just internal refactoring while one adds a new feature.

[ysbaddaden]

Superseded by:

• Add Crystal::EventLoop.current #14559
• Add Fiber.suspend #14560
• Use Fiber#enqueue #14561