Function lambda 

pub fn lambda<F, Fut, I, O>(f: F) -> Lambda<I, O>
where
    F: Fn(I) -> Fut + Send + Sync + 'static,
    Fut: Future<Output = Result<O>> + Send + 'static,
    I: Send + 'static,
    O: Send + 'static,

Convenience constructor — equivalent to Lambda::from_async.