build/cargo-build-system: Make cargo-build-system install working packages.

* guix/build/cargo-build-system.scm (configure): Remove proprietary
dependencies. Add rust dependencies and configure Cargo to find them.
(build): Also build libraries, not just applications.
(file-sha256): New variable.
(generate-checksums): New variable. Export it.
(touch): New variable.
(install): Generate checksums so Cargo accepts the package.