Rewrite traits to use async-fn-in-trait.

- Stub
- BeforeRequest
- AfterRequest

Also removed the last remaining usage of an unstable feature,
iter_intersperse.

example-service/src/lib.rs

@@ -4,9 +4,6 @@
 // license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT.
 
-#![allow(incomplete_features)]
-#![feature(async_fn_in_trait)]
-
 use std::env;
 use tracing_subscriber::{fmt::format::FmtSpan, prelude::*};