Create an ActiveModel::Identity mixin

This commit moves ActiveRecord::Core#== and #hash into a module in ActiveModel.  Any class can now include this mixin to have two instances be considered equal based on their `id`.

I considered making the attribute used for comparison configurable, but it seemed like a lot of complication for little gain.  Sound reasonable?
I don't think it's unclear, but am happy to change it if others feel the same way.

https://en.wikipedia.org/wiki/Identity_(object-oriented_programming)