`tokio::io::ReadHalf<T>::unsplit` is Unsound

Summary

`tokio::io::ReadHalf<T>::unsplit` can violate the `Pin` contract The soundness issue is described in the [tokio/issues#5372](https://github.com/tokio-rs/tokio/issues/5372) Specific set of conditions needed to trigger an issue (a !Unpin type in ReadHalf) is unusual, combined with the difficulty of making any arbitrary use-after-free exploitable in Rust without doing a lot of careful alignment of data types in the surrounding code. The `tokio` feature `io-util` is also required to be enabled to trigger this soundness issue. Thanks to zachs18 reporting the issue to Tokio team responsibly and taiki-e and carllerche appropriately responding and fixing the soundness bug. Tokio before 0.2.0 used `futures` 0.1 that did not have `Pin`, so it is not affected by this issue.