Deprecate `serialized_attributes` without replacement

We've stopped using it internally, in favor of polymorphism. So should
you!
Let's go with an `each_with_object` here, and/or consider memoizing. People obviously should be moving away from it -- that's what the deprecation message is for -- but until then... it used to be super cheap: we probably want to be aware that if people are calling it, they're likely to be doing so with reckless abandon.