Add Fiber::ExecutionContext::Parallel#resize

[ysbaddaden]

Allows to dynamically resize a Parallel execution context to a new maximum parallelism.

Parallelism can grow: merely adds more schedulers, which may eventually start more threads.

Parallelism can shrink: immediately removes any overflow schedulers and tell them to shutdown. The actual shutdown is cooperative, a scheduler won't notice the shutdown until it's current fiber is rescheduled.

Follow up to #15936 and #15946.

[ysbaddaden]

I dropped the Shutdown::NO value and replaced with an invalid value until it's set to Shutdown::NOW. I kept it since @shutdown.now? reads well, but maybe we could drop it, @shutdown_now would read just as well. That depends on the cooperative shutdown.

~~EDIT: I believe the cooperative shutdown would use Shutdown::OnIdle for example. But it could also be a property on Parallel instead of Parallel::Scheduler 🤔 ~~

I dropped the enum and use a boolean that is enough for now. We'll see when we implement context shutdown if an enum would make sense.

I also fixed an issue (must reset @parked) and added an early abort when spinning (no need to spin). I kept the non-blocking evloop run, which doesn't hurt... it might be preferable to enqueue to the global queue directly, though 🤔

[Sija]

I'd use #shutdown? (getter) and #shutdown! to better signify their function. Right now it's IMO kinda unclear with @shutdown ivar and #shutdown method which is not a simple getter.

[ysbaddaden]

• I fixed an issue with the @spinning counter where the counter would go out of sync.
• I renamed the #shutdown method as #shutdown! since it could now be a bit ambiguous compared to the previous #shutdown(:now). I kept @shutdown since it's fully internal to the scheduler.

[straight-shoota]

Could we get a test for ExecutionContext::Parallel? So far this is only testing Runnables.