Remove use of unstable feature 'arbitrary_self_types'.

Turns out, this actually wasn't needed, with some minor refactoring.

example-service/src/client.rs

@@ -4,7 +4,7 @@
 // license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT.
 
-#![feature(arbitrary_self_types, async_await)]
+#![feature(async_await)]
 
 use clap::{App, Arg};
 use futures::{compat::Executor01CompatExt, prelude::*};

example-service/src/lib.rs

@@ -4,7 +4,7 @@
 // license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT.
 
-#![feature(arbitrary_self_types, async_await, proc_macro_hygiene)]
+#![feature(async_await, proc_macro_hygiene)]
 
 // This is the service definition. It looks a lot like a trait definition.
 // It defines one RPC, hello, which takes one arg, name, and returns a String.

example-service/src/server.rs

@@ -4,7 +4,7 @@
 // license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT.
 
-#![feature(arbitrary_self_types, async_await)]
+#![feature(async_await)]
 
 use clap::{App, Arg};
 use futures::{