feat(server): add const_service and service_fn helpers

- `const_service` creates a `NewService` that clones references to the
  wrapped service.
- `service_fn` creates a `Service` from a function. Useful with closures.

src/server/mod.rs

@@ -7,6 +7,7 @@
 mod compat_impl;
 #[cfg(feature = "compat")]
 pub mod compat;
+mod service;
 
 use std::cell::RefCell;
 use std::fmt;
@@ -46,6 +47,8 @@ feat_server_proto! {
     };
 }
 
+pub use self::service::{const_service, service_fn};
+
 /// An instance of the HTTP protocol, and implementation of tokio-proto's
 /// `ServerProto` trait.
 ///