Feature: Return model in persistence

Let the save method return the model to allow for cases like:

```ruby
class SaveModel
  def initialize(model)
    @model = model
  end

  def call
    Model.save do |model|
      model.value = 'test'
    end
  end
end

use_case = SaveModel.new Model.new(id: 5)
model = use_case.call
```