Add a simple method to run a block every X time

[meew0]

I haven't received a response to #4255 yet, so I'm not sure if this method actually belongs in the standard library, but this is how I'd implement it, and I'd certainly use this method a lot in my code. Closes #4255.

[konovod]

What should it do if a task takes more time then given span? In your implementation it will call sleep with negative time, i think it should do something like Fiber.yield or sleep(0) (to prevent monopolization). If sleep with negative time works the same way, no problems.

[meew0]

That's a good point, I hadn't thought of that. sleep with negative time causes an error so I added a specific check.

[kostya]

why not just run yield in spawn? and not calculate negative?

[oprypin]

I would suggest at least tripling all the time spans to even have a chance of this test passing consistently.

[RX14]

Why? Even 1ms is aeons at computer speeds. if it turns out to be a problem it can probably be increased.

[meew0]

It appears to pass very consistently on my system (not a single random failure in 1000 test runs). Is this a problem anywhere?

[RX14]

I was wrong, I tried this on my system and it does fail on relatively minor idle jitter events.

[hugoabonizio]

FWIW, we can keep this feature in a separete lib https://github.com/hugoabonizio/schedule.cr

[asterite]

I don't think this belongs in the std.