Add StreamBody (#237)

This adds `StreamBody` which converts a `Stream` of `Bytes` into a `http_body::Body`.

---

As suggested by Kestrer on Discord it would make sense for axum to provide different kinds of body types other than `Empty`, `Full`, and `hyper::Body`. There is also some talk about [splitting up `hyper::Body`](https://github.com/hyperium/hyper/issues/2345) so this can be seen as getting started on that effort. axum's body types could be moved to hyper or http-body if thats the direction we decide on.

The types I'm thinking about adding are:

- `StreamBody`-  added in this PR
- `AsyncReadBody` - similar to [http-body#41](https://github.com/hyperium/http-body/pull/41/files)
- `ChannelBody` - similar to `hyper::Body::channel`

src/body.rs

@@ -3,6 +3,10 @@
 use crate::BoxError;
 use crate::Error;
 
+mod stream_body;
+
+pub use self::stream_body::StreamBody;
+
 #[doc(no_inline)]
 pub use http_body::{Body as HttpBody, Empty, Full};