Use tokio_util::io::poll_read_buf instead of manual impl

[paolobarbolini]

While auditing usage of bytes I've found this crate manually re-implements tokio_util::io::poll_read_buf. Given that tokio-util is already a dependency, the io feature doesn't import any other dependency, and the manual implementation uses unsafe, I think it'd be worth it to replace this manual implementation with an official one.

[paolobarbolini]

Ping @berkowski @estokes @ColinFinck