Publish new releases

[CleanCut]

We have not published releases, well, ever. To be resolved first:

• Determine whether to create new crates.io entries using the proper names instead of the existing misnamed crate entries on crates.io that were contributed by @mattstam (I think our twirp and twirp-build crates ought to have entries by the same name, not the slightly different entries twirp-rs and twirp-build-rs)
• Select a tool to make multi-crate releases easier
• Establish a release process
• Publish twirp 0.8.0 and twirp-build 0.8.0

[CleanCut]

The twirp and twirp-build names were indeed available on crates.io. I published empty, placeholder versions of twirp and twirp-build on crates.io to reserve the names.

I have looked into four possibilities for managing multi-crate releases to make the crate-publishing process more automated: cargo-release, cargo-workspaces, cargo-smart-release, and release-plz. The one that looks most promising to me is release-plz, since it includes functionality for integrating with Pull Requests and GitHub Actions, which would make it fit into our workflow better than requiring multiple people to maintain CLI environments.