Add Interval

[trowski]

This adds a utility class Interval which controls a repeating timer, automatically cancelling the timer when the object is destroyed.

The main use-case of this object is to store an instance of Interval as an object property. When the holding object is destroyed, the repeating timer will be cancelled. This avoids a bit of boilerplate in the class, such as needing to declare a destructor to manually cancel the event loop callback.

The value here isn't necessarily huge, but this is a pattern we frequently use for classes with self-cleanup mechanisms such as LocalCache.

[xpader]

It's just a auto destruct for EventLoop::repeat()??
I think it's not very necessary for a base library.

[trowski]

How about moving this to amphp/sync? That library has sort of become a utility library.

[kelunik]

I'd rather keep this here, as it's very basic, no?

[trowski]

@kelunik Yeah, I can't see this needing changing, so seems basic enough to go here. Should we add this then?