pub(super) fn unzip<I, A, B, FromA, FromB>(pi: I) -> (FromA, FromB)where
    I: ParallelIterator<Item = (A, B)>,
    FromA: Default + Send + ParallelExtend<A>,
    FromB: Default + Send + ParallelExtend<B>,
    A: Send,
    B: Send,Expand description
Unzips the items of a parallel iterator into a pair of arbitrary
ParallelExtend containers.
This is called by ParallelIterator::unzip.