Change FutureService's associated types to be bounded by IntoFuture rather than Future.

It's strictly more flexible, because everything that impls Future impls IntoFuture, and it
additionally allows returning types like Result. Which is nice.

examples/server_calling_server.rs

@@ -40,10 +40,10 @@ pub mod double {
 struct AddServer;
 
 impl AddFutureService for AddServer {
-    type AddFut = futures::Finished<i32, Never>;
+    type AddFut = Result<i32, Never>;
 
     fn add(&self, x: i32, y: i32) -> Self::AddFut {
-        futures::finished(x + y)
+        Ok(x + y)
     }
 }