Provide async iterator for event loop

[John Hui]

I'm hoping to integrate the piston event loop with some other async code. Is this a feature you would consider supporting?

As a proof-of-concept, I implemented this, with only two lines of change to the next() function; one to replace the use of spin_sleep with tokio::time::sleep, and one to declare the next() function async.

I have two questions about this:

1. Are there any pitfalls I'm missing here?
2. Considering the changes needed are so small, would you consider adding async support to your event_loop, to avoid my duplicating that code?

[Sven Nilsen]

Can you elaborate on the motivation for using an async event loop? Are there any tradeoffs?