Add StreamBody (#237)

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

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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/tests/mod.rs

@@ -701,3 +701,4 @@ where
 
 pub(crate) fn assert_send<T: Send>() {}
 pub(crate) fn assert_sync<T: Sync>() {}
+pub(crate) fn assert_unpin<T: Unpin>() {}