Make middleware::from_fn response future public (#778)

* Make `middleware::from_fn` response future public

* Make module name consistent with tower

axum/src/middleware/mod.rs

@@ -5,3 +5,9 @@
 mod from_fn;
 
 pub use self::from_fn::{from_fn, FromFn, FromFnLayer, Next};
+
+pub mod future {
+    //! Future types.
+
+    pub use super::from_fn::ResponseFuture as FromFnResponseFuture;
+}