Including .clippy.toml in crate tarball leads to older clippy versions failing

… even on crates that have slab as a dependency. [cargo-diet](https://github.com/the-lean-crate/cargo-diet) might help setting a reasonable include / exclude value in `Cargo.toml` / `[package]`. Otherwises [here](https://doc.rust-lang.org/cargo/reference/manifest.html#the-exclude-and-include-fields)'s the docs for those settings.