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`.
Greg Ward, author of the original bfiles extension
Na'Tosha Bard of Unity Technologies
Fog Creek Software
Special thanks to the University of Toronto and the UCOSP program