When the body type of a `Request` or `Response` implements `HttpBody`,
the `Request` or `Response` itself now implements `HttpBody`.
This allows writing things like `hyper::body::aggregate(req)` instead of
`hyper::body::aggregate(req.into_body())`.
Closes#2067
Adds utility functions to `hyper::body` to help asynchronously
collecting all the buffers of some `HttpBody` into one.
- `aggregate` will collect all into an `impl Buf` without copying the
contents. This is ideal if you don't need a contiguous buffer.
- `to_bytes` will copy all the data into a single contiguous `Bytes`
buffer.
