0.15.0: Refactor of list diffing

0.15.0

• Added the Differ interface, as well as EqualityDiffer
• Refactored list diffing into a ListDiffer
• Added concept of ChangeRecord.ANY and ChangeRecord.NONE
  • Low-GC ways to expression "something/nothing" changed
• Refactored ListChangeRecord
  • Added named constructors for common use cases
  • Added equality and hashCode checks
  • Added ListChangeRecord.apply to apply a change record
• Added missing @override annotations to satisfy annotate_overrides