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/static_file_server.rs

@@ -4,7 +4,7 @@
 //! cargo run --example static_file_server
 //! ```
 
-use axum::{prelude::*, routing::nest, service::ServiceExt};
+use axum::{prelude::*, routing::nest};
 use http::StatusCode;
 use std::net::SocketAddr;
 use tower_http::{services::ServeDir, trace::TraceLayer};
@@ -19,12 +19,12 @@ async fn main() {
 
     let app = nest(
         "/static",
-        axum::service::get(ServeDir::new(".").handle_error(|error: std::io::Error| {
+        axum::service::get(ServeDir::new(".")).handle_error(|error: std::io::Error| {
             Ok::<_, std::convert::Infallible>((
                 StatusCode::INTERNAL_SERVER_ERROR,
                 format!("Unhandled internal error: {}", error),
             ))
-        })),
+        }),
     )
     .layer(TraceLayer::new_for_http());