traits for composable async functions.

Examples

use core::convert::Infallible;

use xitca_service::{fn_service, Service, ServiceExt};
// apply middleware to async function as service.
let builder = fn_service(|req: String| async { Ok::<_, Infallible>(req) })
    // a middleware function that has ownership of the argument and output of S as Service
    // trait implementor.
    .enclosed_fn(async |service, req| {
        service.call(req).await.map(|mut res| {
            res.push_str("-dagongren");
            res
        })
    });

// build the composited service.
let service = builder.call(()).await?;

// execute the service function with string argument.
let res = service.call("996".to_string()).await?;

assert_eq!(res, "996-dagongren");

async traits for xitca