[Pitch #2] Async/await

[Lantua]

@xwu

if the argument for async being where it is is that it's part of the type, then it seems weird to allow inference of async in spite of a spelled-out type here where what's actually spelled is a valid but different type (i.e., () /* not async */ -> Int ).

Seems to be the same with throws

let a = { () -> Int in
    throw NSError()
    return 3
}