Function flatten 

pub fn flatten<IntoIter, Si>(sink: Si) -> Flatten<Si, IntoIter>
where
    IntoIter: IntoIterator,
    Si: Sink<IntoIter::Item>,

Creates a Flatten sink that flattens items that are iterators.

Note: Due to type inference limitations, you may need to specify the item type: flatten::<Vec<i32>, _>(sink) where Vec<i32> is the input item type.