feat(body): add body::aggregate and body::to_bytes functions

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.

src/common/mod.rs

@@ -7,6 +7,7 @@ macro_rules! ready {
     };
 }
 
+pub(crate) mod buf;
 pub(crate) mod drain;
 pub(crate) mod exec;
 pub(crate) mod io;