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/body.rs

@@ -1,7 +1,7 @@
 //! HTTP body utilities.
 
+use crate::BoxError;
 use crate::Error;
-use tower::BoxError;
 
 #[doc(no_inline)]
 pub use http_body::{Body as HttpBody, Empty, Full};