Delay of iteration side effects is confusing

This code produces no output:

``` .rs
fn main() {
    let xs = &[1u,2u,3u];
    let f = |x: &uint| {
        println!("{:u}", *x);
    };
    xs.iter().map(f);
}
```

It's because the iterator returned by `map` is never used.  Switching to `.map(f).last()` produces the expected output.

Perhaps iterator structs should be `#[must_use]`?
