The +rustversion call semantics are specific to rustup and do not
work with standard rust toolchains. However, because rustfmt
formatting differs slightly between stable and our 1.63 target, we
need to keep the +1.63.0 for rustup users.
Thus, here, we check for the presence of a `rustup` command and
pass the "+1.63.0" argument if we find one.
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.
