/// Reorder the elements of this iterator *in-place* according to the given predicate,
/// such that all those that return `true` precede all those that return `false`.
/// Returns the number of `true` elements found.
fn partition_in_place<'a, T: 'a, P>(mut self, ref mut predicate: P) -> usize
where
    Self: Sized + DoubleEndedIterator<Item = &'a mut T>,
    P: FnMut(&T) -> bool,