Introduce Adapter::Base

Breaking change:
- Adapters now inherit Adapter::Base
- 'Adapter' is now a module, no longer a class
Why?

- using a class as a namespace that you also inherit from is complicated and circular at time i.e.
  buggy (see https://github.com/rails-api/active_model_serializers/pull/1177)
- The class methods on Adapter aren't necessarily related to the instance methods, they're more
    Adapter functions
- named `Base` because it's a Rails-ism
- It helps to isolate and highlight what the Adapter interface actually is

test/adapter/fragment_cache_test.rb

@@ -1,7 +1,7 @@
 require 'test_helper'
 module ActiveModel
   class Serializer
-    class Adapter
+    module Adapter
       class FragmentCacheTest < Minitest::Test
         def setup
           @spam            = Spam::UnrelatedLink.new(id: 'spam-id-1')

test/adapter/json/belongs_to_test.rb

@@ -2,7 +2,7 @@ require 'test_helper'
 
 module ActiveModel
   class Serializer
-    class Adapter
+    module Adapter
       class Json
         class BelongsToTest < Minitest::Test
           def setup

test/adapter/json/collection_test.rb

@@ -2,7 +2,7 @@ require 'test_helper'
 
 module ActiveModel
   class Serializer
-    class Adapter
+    module Adapter
       class Json
         class Collection < Minitest::Test
           def setup

test/adapter/json/has_many_test.rb

@@ -2,7 +2,7 @@ require 'test_helper'
 
 module ActiveModel
   class Serializer
-    class Adapter
+    module Adapter
       class Json
         class HasManyTestTest < Minitest::Test
           def setup

test/adapter/json_api/belongs_to_test.rb

@@ -2,7 +2,7 @@ require 'test_helper'
 
 module ActiveModel
   class Serializer
-    class Adapter
+    module Adapter
       class JsonApi
         class BelongsToTest < Minitest::Test
           def setup

test/adapter/json_api/collection_test.rb

@@ -2,7 +2,7 @@ require 'test_helper'
 
 module ActiveModel
   class Serializer
-    class Adapter
+    module Adapter
       class JsonApi
         class CollectionTest < Minitest::Test
           def setup

test/adapter/json_api/has_many_embed_ids_test.rb

@@ -2,7 +2,7 @@ require 'test_helper'
 
 module ActiveModel
   class Serializer
-    class Adapter
+    module Adapter
       class JsonApi
         class HasManyEmbedIdsTest < Minitest::Test
           def setup

test/adapter/json_api/has_many_explicit_serializer_test.rb

@@ -2,7 +2,7 @@ require 'test_helper'
 
 module ActiveModel
   class Serializer
-    class Adapter
+    module Adapter
       class JsonApi
         # Test 'has_many :assocs, serializer: AssocXSerializer'
         class HasManyExplicitSerializerTest < Minitest::Test

test/adapter/json_api/has_many_test.rb

@@ -2,7 +2,7 @@ require 'test_helper'
 
 module ActiveModel
   class Serializer
-    class Adapter
+    module Adapter
       class JsonApi
         class HasManyTest < Minitest::Test
           def setup

test/adapter/json_api/has_one_test.rb

@@ -2,7 +2,7 @@ require 'test_helper'
 
 module ActiveModel
   class Serializer
-    class Adapter
+    module Adapter
       class JsonApi
         class HasOneTest < Minitest::Test
           def setup

test/adapter/json_api/json_api_test.rb

@@ -2,7 +2,7 @@ require 'test_helper'
 
 module ActiveModel
   class Serializer
-    class Adapter
+    module Adapter
       class JsonApiTest < Minitest::Test
         def setup
           ActionController::Base.cache_store.clear

test/adapter/json_api/linked_test.rb

@@ -1,7 +1,7 @@
 require 'test_helper'
 module ActiveModel
   class Serializer
-    class Adapter
+    module Adapter
       class JsonApi
         class LinkedTest < Minitest::Test
           def setup

test/adapter/json_api/pagination_links_test.rb

@@ -6,7 +6,7 @@ require 'kaminari/hooks'
 
 module ActiveModel
   class Serializer
-    class Adapter
+    module Adapter
       class JsonApi
         class PaginationLinksTest < Minitest::Test
           URI = 'http://example.com'

test/adapter/json_api/resource_type_config_test.rb

@@ -2,7 +2,7 @@ require 'test_helper'
 
 module ActiveModel
   class Serializer
-    class Adapter
+    module Adapter
       class JsonApi
         class ResourceTypeConfigTest < Minitest::Test
           def setup

test/adapter/json_test.rb

@@ -2,7 +2,7 @@ require 'test_helper'
 
 module ActiveModel
   class Serializer
-    class Adapter
+    module Adapter
       class JsonTest < Minitest::Test
         def setup
           ActionController::Base.cache_store.clear

test/adapter/null_test.rb

@@ -2,7 +2,7 @@ require 'test_helper'
 
 module ActiveModel
   class Serializer
-    class Adapter
+    module Adapter
       class NullTest < Minitest::Test
         def setup
           profile = Profile.new({ name: 'Name 1', description: 'Description 1', comments: 'Comments 1' })