Rework automatic :id assignment

[nbibler]

In the Mapping definition the :id attribute is automatically defined. While I recognize that JSON API requires an ID, I'm not entirely sure that AMS does. Further, recent changes to the serializers to allow for customizations of the value (block attribute syntax, for example) cannot be used with this attribute definition.

# /lib/ivy/serializers/mapping.rb
module Ivy
  module Serializers
    class Mapping
      def initialize(klass)
        # ...

        attribute(:id)
      end
    end
  end
end

Should the :id continue to be auto-set or should that be the responsibility of the developer to include in each/all of their serializers?