Attach quick fixes to warning and error messages

This fixes #92 (closed) by attaching a list of frontend-provided quick fixes to each warning/error message, e.g. for use in the language server. As an initial example, this adds such a fix for the warning suggesting a missing top-level type signature:

Future PRs could add further fixes (e.g. from the suggestions in #92 (closed)) and we could potentially also integrate the fixes into the prettyprinted messages emitted by the frontend itself (e.g. by visualizing the fix in the span preview).