Show diff for long predicate failures

[brandonchinn178]

Currently, failures can look like

[User "Alice" 30, User "Bob" 20, User "Carla" 10] ≠ [User "Alice" 30, User "Carla" 20, User "Devon" 10]

It would look better to add some basic diff support, like

[User "Alice" 30, User "Bob" 20, User "Carla" 10] ≠ [User "Alice" 30, User "Carla" 20]

[
  User "Alice" 30,
- User "Bob" 20,
- User "Carla" 10,
+ User "Carla" 20,
]