* guix/build/cargo-build-system.scm (generate-all-checksums): New
procedure.
(update-cargo-lock, patch-cargo-checksums): New phases.
(%standard-phases): Add 'update=cargo-lock after 'configure and
'patch-cargo-checksums after 'patch-generated-file-shebangs.
* doc/guix.texi (Build System)[cargo-build-system]: Mention how
Cargo.lock files are handled.