Consider switching to insta for snapshot testing

[obi1kenobi]

This project relies heavily on snapshot testing: we write test crates and lint them, then ensure that the outputs are both expected and haven't changed since the last run.

When cargo-semver-checks came into existence, insta wasn't nearly as polished as it's become since. These days, it's feature-rich and fairly commonly used in the ecosystem.

Our own hand-rolled snapshot testing can be a bit annoying to manage: one has to manually edit snapshot files, and observe proper item order since they are order-sensitive. However, it also allows us some conveniences: we can leave comments inside snapshot files (e.g. look at the removed lines in #631 for a useful snapshot comment), and we have tight control over how information is stored and where.

Can an insta-based workflow replicate the conveniences we care about, or offer other conveniences that make switching to it compelling?

To close this issue, we need to:

• figure out what a good insta-based workflow would look like
• consider whether it's better than what we have right now -- and if so
  • replace our snapshot testing with insta
  • update our CONTRIBUTING doc with the new workflow, making sure to add guidance on how to install cargo-insta and how to use insta and cargo insta productively