Re-export `Bytes` struct

[nwin]

Currently bytes::bytes::Bytes is used in a public API (e.g. Response::bytes). This struct is inaccessible without adding bytes as a dependency to Cargo.toml and thus should be re-exported.

I could file a PR if you advise where an how it should be re-exported.

[bluelhf]

Since this issue has been stale for almost ten months now, I've gone ahead and opened a pull request that should resolve the issue :)