Function eventuals::map_with_retry

pub fn map_with_retry<I, Ok, Err, F, Fut, E, FutE>(
    source: Eventual<I>, 
    f: F, 
    on_err: E
) -> Eventual<Ok> where
    F: 'static + Clone + Send + Fn(I) -> Fut,
    E: 'static + Clone + Send + Sync + Fn(Err) -> FutE,
    I: Value,
    Ok: Value,
    Err: Value,
    Fut: Send + Future<Output = Result<Ok, Err>>,
    FutE: Send + Future<Output = ()>,