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/extract/connect_info.rs

@@ -14,8 +14,8 @@ use std::{
     net::SocketAddr,
     task::{Context, Poll},
 };
-use tower::Service;
 use tower_http::add_extension::AddExtension;
+use tower_service::Service;
 
 /// A [`MakeService`] created from a router.
 ///

src/extract/extractor_middleware.rs

@@ -3,6 +3,7 @@
 //! See [`extractor_middleware`] for more details.
 
 use super::{FromRequest, RequestParts};
+use crate::BoxError;
 use crate::{body::BoxBody, response::IntoResponse};
 use bytes::Bytes;
 use futures_util::{future::BoxFuture, ready};
@@ -15,7 +16,8 @@ use std::{
     pin::Pin,
     task::{Context, Poll},
 };
-use tower::{BoxError, Layer, Service};
+use tower_layer::Layer;
+use tower_service::Service;
 
 /// Convert an extractor into a middleware.
 ///

src/extract/form.rs

@@ -1,10 +1,10 @@
 use super::{has_content_type, rejection::*, take_body, FromRequest, RequestParts};
+use crate::BoxError;
 use async_trait::async_trait;
 use bytes::Buf;
 use http::Method;
 use serde::de::DeserializeOwned;
 use std::ops::Deref;
-use tower::BoxError;
 
 /// Extractor that deserializes `application/x-www-form-urlencoded` requests
 /// into some type.

src/extract/multipart.rs

@@ -3,6 +3,7 @@
 //! See [`Multipart`] for more details.
 
 use super::{rejection::*, BodyStream, FromRequest, RequestParts};
+use crate::BoxError;
 use async_trait::async_trait;
 use bytes::Bytes;
 use futures_util::stream::Stream;
@@ -13,7 +14,6 @@ use std::{
     pin::Pin,
     task::{Context, Poll},
 };
-use tower::BoxError;
 
 /// Extractor that parses `multipart/form-data` requests commonly used with file uploads.
 ///

src/extract/rejection.rs

@@ -1,6 +1,7 @@
 //! Rejection response types.
 
 use super::IntoResponse;
+use crate::BoxError;
 use crate::{
     body::{box_body, BoxBody},
     Error,
@@ -8,7 +9,6 @@ use crate::{
 use bytes::Bytes;
 use http_body::Full;
 use std::convert::Infallible;
-use tower::BoxError;
 
 define_rejection! {
     #[status = INTERNAL_SERVER_ERROR]

src/extract/request_parts.rs

@@ -1,4 +1,5 @@
 use super::{rejection::*, take_body, Extension, FromRequest, RequestParts};
+use crate::BoxError;
 use async_trait::async_trait;
 use bytes::Bytes;
 use futures_util::stream::Stream;
@@ -8,7 +9,6 @@ use std::{
     pin::Pin,
     task::{Context, Poll},
 };
-use tower::BoxError;
 
 #[async_trait]
 impl<B> FromRequest<B> for Request<B>