makefile: add `cargo clippy` to tests if cargo is available This linter mostly makes our code more idiomatic, less surprising, has good suggestions and catches bugs. It's widely used in the Rust community and now part of the default toolchain when using `rustup`.

rust-tests: run `cargo` tests inside the `rust` folder There is no reason to only run inside the `rust/hg-cpython` folder, even if the `cargo test --all` behavior ends up being the same.

rust-clippy: merge "revlog" module definition and struct implementation Module inception is confusing when looking for code and almost never the right choice.

rust-clippy: merge "config" module definition and struct implementation Module inception is confusing when looking for code and almost never the right choice.

rust-clippy: upgrade `bytes-cast` dependency I released a new version of `bytes-cast` to get rid of the clippy warning, and bump to edition 2021, so let's use it.

rust-clippy: remove redundant suffix from enum Same as last time, this makes the code clearer in this instance.