Function function_stage 

pub fn function_stage<F, I, O>(name: &str, f: F) -> Box<dyn PipelineStage>
where
    F: Fn(I) -> Result<O> + Send + Sync + 'static,
    I: 'static + Send + Sync,
    O: 'static + Send + Sync,

Create a simple function-based pipeline stage