Peekable's is_empty() has the wrong definition.

An iterator returns None if it is empty, but Peekable's is_empty is defined like this:

```
pub fn is_empty(&mut self) -> bool {
    self.peek().is_some()
}
```

https://github.com/mozilla/rust/blob/master/src/libstd/iter.rs#L1377

This needs to be changed to use is_none().

In fact, the code from test_peekable_is_empty()  fails. It looks like this escaped detection because of a missing "#[test]"
