Support returning any http_body::Body from IntoResponse (#86)

Adds associated `Body` and `BodyError` types to `IntoResponse`. This is required for returning responses with bodies other than `hyper::Body` from handlers. That wasn't previously possible.

This is a breaking change so should be shipped in 0.2.

src/lib.rs

@@ -707,7 +707,6 @@
 #![cfg_attr(docsrs, feature(doc_cfg))]
 #![cfg_attr(test, allow(clippy::float_cmp))]
 
-use self::body::Body;
 use http::Request;
 use routing::{EmptyRouter, Route};
 use tower::Service;