Refactor Fix and Edit API

[MichaReiser]

This issue is part of #4181. The goal is to prepare the Fix and Edit API to ease adding the Applicability to Fix in a follow-up PR.

I propose the following changes to the API. Please feel free to deviate from or extend the proposal if I overlooked something.

• Diagnostic:
  • Change fix to Option<Fix>
  • Change set_fix, with_fix and try_set_fix to accept a Fix
• Fix:
  • Rename new to unspecified
  • Create the new unspecified_edits(edit: Edit, rest: impl IntoIterator<Item = Edit>) -> Self factory function
  • Change unspecified to unspecified(edit: Edit) -> Self
  • Remove is_empty(),
  • Remove the FromIterator, From<Edit> and Default implementations
  • Maybe: Add a push_edit(&mut self, edit; Edit) method