Avoid introducing dependencies that break `no_std`

I believe it's possible to accidentally break downstream consumers of a supposedly `no_std` crate by adding a dependency that is not itself `no_std`. Prevent this with a check on GitHub actions. This can be done with `cargo nono check`, or perhaps a `no_std` test crate.

Start by reviewing the repository's GitHub Actions configuration and compare the proposed `cargo nono check` approach with a dedicated `no_std` test crate. Done means CI detects dependencies that break the crate's `no_std` guarantee without affecting valid builds.

Written by the indexing model from the issue text.