Based on #533 and work for mozilla/glean#2272.
It would be nice to have that land when we land 2272 as well, but it's not necessary. The old way still works and FOG does its own thing anyway (though it can do it better with this).

This allows us to generate non-generic code to turn strings into the correct type.

Pull Request checklist

• Quality: This PR builds and tests run cleanly
  • make test runs without emitting any warnings
  • make lint runs without emitting any errors
• Tests: This PR includes thorough tests or an explanation of why it does not
• Changelog: This PR includes a changelog entry to CHANGELOG.md or an explanation of why it does not need one
  • Any breaking changes to language binding APIs are noted explicitly