Update the tokio-util dev-dependency to 0.7.x

[musicinmybrain]

The goal here is just to be SemVer-compatible with the current release rather than pinning an old release for no particular reason.

https://github.com/tokio-rs/tokio/blob/master/tokio-util/CHANGELOG.md

[HeroicKatora]

Thank you. On a general note, before you worry too much about it, I'm fine with the idea of bumping MSRV from the current rather ancient 1.34 to something that at least supports cargo's rust-version mechanism in Cargo.toml appropriately. While I'm not sure if this should be released as 0.2 or just a new minor version, the major reason was compatibility with migration from the lzw crate. This should no longer be a concern warranting holding the msrv back so much.