Skip should implement ExactSizeIterator

This should be in `iter.rs`:

``` rust
impl<I> ExactSizeIterator for Skip<I> where I: ExactSizeIterator{ }
```

This would require either implementing `DoubleEndedIterator` for `Skip<I>` (where `I: DoubleEndedIterator`), or decoupling `ExactSizeIterator` and `DoubleEndedIterator` (like in #19395)
