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?
Do you think `IdComparison` would be a better name than `Identity`? To me, `Identity` implies that this has something to do with the identity function, which this does not.