CI build should fail, if crates are not part of workspace

[hannobraun]

As of #475 crates are organized into different directories. All crates that are targeted at ecosystem or end users are in crates/, internal tools are in tools/, and as before, all models are still in models/.

There are some rules that should be checked as part of the CI build, to prevent mistakes:

1. All crates from all directories should be part of the workspace, i.e. be in the workspace.members list in Cargo.toml.
2. Crates from the crates/ directory should in addition be part of the workspace.default-members list.

This could be added to the CI configuration as an additional job. That job can run only on Linux, like the Format and Clippy Check jobs. If this isn't easy to do only with GitHub Actions, we can add a new Rust program to tools/, and call that from the job using actions-rs/cargo (other jobs are already doing this).

Labeling as https://github.com/hannobraun/Fornjot/labels/good%20first%20issue, since this doesn't require lots of knowledge of Fornjot, and could serve as an introduction to the project.

Related issue: #478

[hannobraun]

I've decided to close this. We don't add or remove crates often enough that it's worth putting the effort into custom tooling.