This is the first PR of a WIP to bring the attributes API to
ActiveModel. It is not yet ready for public API.
The `attributes_dirty_test.rb` file was created based on `dirty_test.rb`,
and the simplifications in the diff do much to motivate this change.
```
diff activemodel/test/cases/dirty_test.rb activemodel/test/cases/attributes_dirty_test.rb
3a4
> require "active_model/attributes"
5c6
< class DirtyTest < ActiveModel::TestCase
---
> class AttributesDirtyTest < ActiveModel::TestCase
7,41c8,12
< include ActiveModel::Dirty
< define_attribute_methods :name, :color, :size
<
< def initialize
< @name = nil
< @color = nil
< @size = nil
< end
<
< def name
< @name
< end
<
< def name=(val)
< name_will_change!
< @name = val
< end
<
< def color
< @color
< end
<
< def color=(val)
< color_will_change! unless val == @color
< @color = val
< end
<
< def size
< @size
< end
<
< def size=(val)
< attribute_will_change!(:size) unless val == @size
< @size = val
< end
---
> include ActiveModel::Model
> include ActiveModel::Attributes
> attribute :name, :string
> attribute :color, :string
> attribute :size, :integer
```
