Allow using async in for loops over collections

Hi,

I think in case of (sequential) collections this code

``` scala
for (x ← xs) {
  ... async ...
}
```

could be rewritten into

``` scala
val it = xs.iterator
while (it.hasNext) {
  val x = it.next()
  ... async ...
}
```

thus relaxing the restriction regarding `async` inside closures. It only saves a few lines, but looks more idiomatic. What do you think?
