New release process (release with a manually triggered action)

[shnewto]

This PR adds an action that can be manually triggered to handle bumping the Cargo.toml version, adding a corresponding tag to the repo, and publishing to crates.io. The full proposed process is documented in the RELEASE.md that's been added here too.

Some things I thought about then decided against before settling on this solution:

• We could potentially add another step to the action to actually create a release as well but I opted not to in order to leave some room for the kinds of unautomatable notes that should sometimes make it into a release.
• We could publish to crates automatically after creating a release manually, but then we end up needing to manually edit the version in the Cargo.toml in order for it to be in the state of code covered in the release's tag. I don't like having to remember to change the version in the Cargo.toml though, it's the part I most often forget and then trip over.

(There are also a couple fixes for clippy lints that made it in here and some fixes to other errors that showed up in CI)

[coveralls]

coverage: 96.67% (+0.08%) from 96.589% when pulling 6c2a92554bd652c26581a09b827b521d2700a1e4 on publish-crate-action into 465b7977ca63f81c818670250ec4d8968bb0e425 on main.

[shnewto]

Oh something maybe notable in the implementation you might catch: runs-on: macos-latest. This is something I've adopted as a default instead of ubuntu-latest in other crates because I don't have as easy access to an ubuntu machine to troubleshoot issues, and macos-latest also seems to less often need preliminary sudo apt ... steps to pull in extra system dependencies.