Move execution context event loop lock to each event loop

[ysbaddaden]

This is a requirement for io_uring, and it improves readability despite a slight duplication.

Extracted from #16264

[straight-shoota]

Placing the lock on the event loop looks good.
But I'm wondering if we should move the implementation to EventLoop#lock? since it's shared across most subclasses. It's trivial to override with a no-op for io_uring.
That would put an unused @lock ivar in EventLoop::IOUring though 🤷
So maybe this is just fine.

[ysbaddaden]

There could be a Crystal::EventLoop::Lock module that would implement the abstract methods a the general lock. That would avoid the duplication (and the override). Then io_uring would simply implement the methods.

[ysbaddaden]

Fixed conflicts with the master branch and added Lock module.