Use #[allow(unreachable_patterns)] to simplify some macro code. (#163)

By allowing unreachable patterns, we don't have to have 'NotIrrefutable' variants in the Request, Response, and Error enums.

This commit also removes an unused macro in an example.

examples/two_clients.rs

@@ -57,10 +57,6 @@ impl baz::FutureService for Baz {
     }
 }
 
-macro_rules! pos {
-    () => (concat!(file!(), ":", line!()))
-}
-
 fn main() {
     let _ = env_logger::init();
     let bar_client = {