Part of #4181 and depends on #4182

• Add a new Applicability enum in fix.rs similar to Rome's or Clippy's Applicability enums with three variants:
  • Safe (or Automatic, Always, MachineApplicable?)
  • MaybeIncorrect
  • Unspecified
• Add the applicability field to Fix
• Add the new safe and safe_edits constructor functions (identical to unspecified and unspecified_edits)
• Add the new maybe_incorrect and maybe_incorrect_edits constructor functions (identical to unspecified and unspecified_edits)
• Mark unspecified and unspecified_edits as deprecated (new lints should use either safe_* or maybe_incorrect_*). Add allow(deprecated)` at the call-sites.
• Change the Diff implementation to write Suggested Fix for MaybeIncorrect and Unspecified, and Safe fix for Applicability::Safe.