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 guess depends on what is the objective of ActiveModelout of the box. If the minimum to make it behave like a model for form purposes, then no. If supporting comparison/sorting, then yes.