Revert "Implement ArraySerializer and move old serialization API to a new namespace."

This reverts commit 8896b4fdc8.

Conflicts:

	activemodel/lib/active_model.rb
	activemodel/lib/active_model/serializable.rb
	activemodel/lib/active_model/serializer.rb
	activemodel/test/cases/serializer_test.rb

activemodel/test/cases/serialization_test.rb

@@ -3,7 +3,7 @@ require 'active_support/core_ext/object/instance_variables'
 
 class SerializationTest < ActiveModel::TestCase
   class User
-    include ActiveModel::Serializable
+    include ActiveModel::Serialization
 
     attr_accessor :name, :email, :gender, :address, :friends
 
@@ -22,7 +22,7 @@ class SerializationTest < ActiveModel::TestCase
   end
 
   class Address
-    include ActiveModel::Serializable
+    include ActiveModel::Serialization
 
     attr_accessor :street, :city, :state, :zip
 

activemodel/test/cases/serializers/json_serialization_test.rb

@@ -5,7 +5,7 @@ require 'active_support/core_ext/object/instance_variables'
 
 class Contact
   extend ActiveModel::Naming
-  include ActiveModel::Serializable::JSON
+  include ActiveModel::Serializers::JSON
   include ActiveModel::Validations
 
   def attributes=(hash)

activemodel/test/cases/serializers/xml_serialization_test.rb

@@ -5,7 +5,7 @@ require 'ostruct'
 
 class Contact
   extend ActiveModel::Naming
-  include ActiveModel::Serializable::XML
+  include ActiveModel::Serializers::Xml
 
   attr_accessor :address, :friends
 
@@ -26,7 +26,7 @@ end
 
 class Address
   extend ActiveModel::Naming
-  include ActiveModel::Serializable::XML
+  include ActiveModel::Serializers::Xml
 
   attr_accessor :street, :city, :state, :zip