* guix/build-system/cargo.scm (cargo-build): Add cargo-package-flags,
install-source flags.
* guix/build/cargo-build-system.scm (unpack-rust-crates, package): New
procedures.
(install): Also install crate sources.
(%standard-phases): Add new phases.
* doc/guix.texi (Packaging-guidelines)[Rust Crates]: Adjust to changes
in the cargo-build-system.