Function map_reduce 

pub fn map_reduce<'a, T, S, FM, FR, FD, R>(
    source: &'a S,
    map_op: FM,
    default_op: FD,
    reduce_op: FR,
) -> R
where
    T: Send + Sync,
    S: IntoParallelRefIterator<'a, Item = T> + ?Sized,
    FM: Fn(T) -> R + Sync + Send,
    FR: Fn(R, R) -> R + Sync + Send,
    FD: Fn() -> R + Sync + Send,
    R: Send,

Performs map reduce operations in parallel.