Add service name to generated items.

With this change, the service definitions don't need to be isolated in their own modules.

example-service/src/lib.rs

@@ -9,7 +9,7 @@
 /// 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.
 #[tarpc::service]
-pub trait Service {
+pub trait World {
     /// Returns a greeting for name.
     async fn hello(name: String) -> String;
 }