Support for await

[KathleenDollard]

I don't support await yet because I can't figure out the semantics, and you actually have two visions in your head depending.

What's really happening is more or less this

// code await stuff { // more code }

but what it looks like is

// code await stuff // more code

The simplicity of the second version was core to the design of async. However, I also think it trips people up and that the first approach will make it much easier to do certain types of analysis.

I have three design ideas

• nest statements following await, per first code above
• have await be a random keyword like continue
• provide two "modes", async simple, async expanded

I really, really hate the idea of modes. But I see such serious problems with both of the other options I'm leaning to it.