Move methods from ServiceExt to RoutingDsl (#160)

Previously, on `main`, this wouldn't compile:

```rust
let app = route("/", get(handler))
    .layer(
        ServiceBuilder::new()
            .timeout(Duration::from_secs(10))
            .into_inner(),
    )
    .handle_error(...)
    .route(...); // <-- doesn't work
```

That is because `handle_error` would be
`axum::service::ServiceExt::handle_error` which returns `HandleError<_,
_, _, HandleErrorFromService>` which does _not_ implement `RoutingDsl`.
So you couldn't call `route`. This was caused by
https://github.com/tokio-rs/axum/pull/120.

Basically `handle_error` when called on a `RoutingDsl`, the resulting
service should also implement `RoutingDsl`, but if called on another
random service it should _not_ implement `RoutingDsl`.

I don't think thats possible by having `handle_error` on `ServiceExt`
which is implemented for any service, since all axum routers are also
services by design.

This resolves the issue by removing `ServiceExt` and moving its methods
to `RoutingDsl`. Then we have more tight control over what has a
`handle_error` method.

`service::OnMethod` now also has a `handle_error` so you can still
handle errors from random services, by doing
`service::any(svc).handle_error(...)`.

examples/websocket.rs

@@ -14,7 +14,6 @@ use axum::{
     prelude::*,
     response::IntoResponse,
     routing::nest,
-    service::ServiceExt,
 };
 use http::StatusCode;
 use std::net::SocketAddr;
@@ -35,15 +34,14 @@ async fn main() {
     let app = nest(
         "/",
         axum::service::get(
-            ServeDir::new("examples/websocket")
-                .append_index_html_on_directories(true)
-                .handle_error(|error: std::io::Error| {
-                    Ok::<_, std::convert::Infallible>((
-                        StatusCode::INTERNAL_SERVER_ERROR,
-                        format!("Unhandled internal error: {}", error),
-                    ))
-                }),
-        ),
+            ServeDir::new("examples/websocket").append_index_html_on_directories(true),
+        )
+        .handle_error(|error: std::io::Error| {
+            Ok::<_, std::convert::Infallible>((
+                StatusCode::INTERNAL_SERVER_ERROR,
+                format!("Unhandled internal error: {}", error),
+            ))
+        }),
     )
     // routes are matched from bottom to top, so we have to put `nest` at the
     // top since it matches all routes