There should be a way to count backwards with a step using a range

```
fn main() {
    for i in (0..10).step_by(2).rev() {
        println!("{}", i);
    }
}
```

produces the following error:

```
error: the trait `core::iter::DoubleEndedIterator` is not implemented for the type `core::iter::StepBy<_, core::ops::Range<_>>` [E0277]
```

Perhaps `step_by()` could produce a DoubleEndedIterator on ranges that are double-ended, but I don't know if this is possible.
