Add `rustfmt` to CI, ignore all files

Add `rustfmt` to CI, ignore all files

We add the previously discussed `rustfmt.toml` and enforce it in CI for
any files that are not contained in an exclusion list.

To start, we add all current Rust files to this exclusion list. This
means that formatter rules will be enforced for any newly introduced
files, and we'll then start going through the codebase file-by-file,
removing them from the list as we go.