Incorporate rustfmt into project linting

[thundergolfer]

Description

Thanks to bazelbuild/rules_rust@fe50d3b#diff-d5825ae12e45363c451d288f385bfda1L147 rustfmt is now easily available in the WORKSPACE. See:

bazel run @rust_darwin_x86_64//:rustfmt -- --help

Will have to use a select to handle the fact that the external repo will be different on CI (Linux).