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

- Stub
- BeforeRequest
- AfterRequest

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

plugins/tests/server.rs

@@ -1,6 +1,3 @@
-#![allow(incomplete_features)]
-#![feature(async_fn_in_trait)]
-
 // these need to be out here rather than inside the function so that the
 // assert_type_eq macro can pick them up.
 #[tarpc::service]

plugins/tests/service.rs

@@ -1,6 +1,3 @@
-#![allow(incomplete_features)]
-#![feature(async_fn_in_trait)]
-
 use tarpc::context;
 
 #[test]