tokio::io::ReadHalf<T>::unsplit can violate the Pin contract

The soundness issue is described in the 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.

References

• tokio-rs/tokio#5372
• https://rustsec.org/advisories/RUSTSEC-2023-0005.html