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.

Cargo.toml

@@ -98,7 +98,7 @@ required-features = ["runtime"]
 [[example]]
 name = "client_json"
 path = "examples/client_json.rs"
-required-features = ["runtime", "stream"]
+required-features = ["runtime"]
 
 [[example]]
 name = "echo"
@@ -162,6 +162,11 @@ path = "examples/web_api.rs"
 required-features = ["runtime", "stream"]
 
 
+[[bench]]
+name = "body"
+path = "benches/body.rs"
+required-features = ["runtime", "stream"]
+
 [[bench]]
 name = "connect"
 path = "benches/connect.rs"