Remove tower from axum's public API (#229)

Instead rely on `tower-service` and `tower-layer`. `tower` itself is
only used internally.

Fixes https://github.com/tokio-rs/axum/issues/186

src/handler/future.rs

@@ -16,7 +16,8 @@ use std::{
     pin::Pin,
     task::{Context, Poll},
 };
-use tower::{util::Oneshot, Service};
+use tower::util::Oneshot;
+use tower_service::Service;
 
 pin_project! {
     /// The response future for [`OnMethod`](super::OnMethod).

src/handler/into_service.rs

@@ -7,7 +7,7 @@ use std::{
     marker::PhantomData,
     task::{Context, Poll},
 };
-use tower::Service;
+use tower_service::Service;
 
 /// An adapter that makes a [`Handler`] into a [`Service`].
 ///

src/handler/mod.rs

@@ -7,6 +7,7 @@ use crate::{
     routing::{EmptyRouter, MethodFilter},
     service::HandleError,
     util::Either,
+    BoxError,
 };
 use async_trait::async_trait;
 use bytes::Bytes;
@@ -18,7 +19,9 @@ use std::{
     marker::PhantomData,
     task::{Context, Poll},
 };
-use tower::{BoxError, Layer, Service, ServiceExt};
+use tower::ServiceExt;
+use tower_layer::Layer;
+use tower_service::Service;
 
 pub mod future;
 mod into_service;