`stringifier` is a noop by default, but it is there to allow the caller of Differ to add custom diff support for some objects. See rspec-expectations for an example: https://github.com/rspec/rspec-expectations/pull/441/files#diff-9fc62637ddda0450632b589033415d37R9

Perhaps instead of being a lambda or block it should be a method, with the idea that Differ can be subclassed? A rough sketch might look like:

```
class RSpec::Support::Differ
  # ...

  # noop by defautl
  def stringify_object(object)
    object
  end
  # ...
end

class RSpec::Expectations::Differ < RSpec::Support::Differ
  def stringify_object(object)
    # rspec-expectations specific code.
  end
end
```