Revert "Simplify handler trait (#221)" (#223)

This reverts commit 44c58bdf5f.

src/handler/future.rs

@@ -2,7 +2,10 @@
 
 use crate::body::{box_body, BoxBody};
 use crate::util::{Either, EitherProj};
-use futures_util::{future::BoxFuture, ready};
+use futures_util::{
+    future::{BoxFuture, Map},
+    ready,
+};
 use http::{Method, Request, Response};
 use http_body::Empty;
 use pin_project_lite::pin_project;
@@ -64,5 +67,8 @@ where
 opaque_future! {
     /// The response future for [`IntoService`](super::IntoService).
     pub type IntoServiceFuture =
-        BoxFuture<'static, Result<Response<BoxBody>, Infallible>>;
+        Map<
+            BoxFuture<'static, Response<BoxBody>>,
+            fn(Response<BoxBody>) -> Result<Response<BoxBody>, Infallible>,
+        >;
 }