Move generated functions under their corresponding items.

- fn serve -> Service::serve
- fn new_stub -> Client::new

This allows the generated function names to remain consistent across
service definitions while preventing collisions.

example-service/src/client.rs

@@ -44,9 +44,9 @@ async fn main() -> io::Result<()> {
 
     let transport = json_transport::connect(&server_addr).await?;
 
-    // world_stub is generated by the service attribute. Like Server, it takes a config and any
-    // Transport as input, and returns a Client, also generated by the attribute.
-    let mut client = service::world_stub(client::Config::default(), transport).await?;
+    // WorldClient is generated by the service attribute. It has a constructor `new` that takes a
+    // config and any Transport as input.
+    let mut client = service::WorldClient::new(client::Config::default(), transport).await?;
 
     // The client has an RPC method for each RPC defined in the annotated trait. It takes the same
     // args as defined, with the addition of a Context, which is always the first arg. The Context