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/serializers/adapter_for_test.rb

@@ -127,7 +127,7 @@ module ActiveModel
       def test_inherited_adapter_hooks_register_adapter
         Object.const_set(:MyAdapter, Class.new)
         my_adapter = MyAdapter
-        ActiveModel::Serializer::Adapter.inherited(my_adapter)
+        ActiveModel::Serializer::Adapter::Base.inherited(my_adapter)
         assert_equal ActiveModel::Serializer::Adapter.lookup(:my_adapter), my_adapter
       ensure
         ActiveModel::Serializer::Adapter.adapter_map.delete('my_adapter'.freeze)
@@ -138,7 +138,7 @@ module ActiveModel
         Object.const_set(:MyNamespace, Module.new)
         MyNamespace.const_set(:MyAdapter, Class.new)
         my_adapter = MyNamespace::MyAdapter
-        ActiveModel::Serializer::Adapter.inherited(my_adapter)
+        ActiveModel::Serializer::Adapter::Base.inherited(my_adapter)
         assert_equal ActiveModel::Serializer::Adapter.lookup(:'my_namespace/my_adapter'), my_adapter
       ensure
         ActiveModel::Serializer::Adapter.adapter_map.delete('my_namespace/my_adapter'.freeze)
@@ -151,8 +151,8 @@ module ActiveModel
         my_adapter = MyAdapter
         Object.const_set(:MySubclassedAdapter, Class.new(MyAdapter))
         my_subclassed_adapter = MySubclassedAdapter
-        ActiveModel::Serializer::Adapter.inherited(my_adapter)
-        ActiveModel::Serializer::Adapter.inherited(my_subclassed_adapter)
+        ActiveModel::Serializer::Adapter::Base.inherited(my_adapter)
+        ActiveModel::Serializer::Adapter::Base.inherited(my_subclassed_adapter)
         assert_equal ActiveModel::Serializer::Adapter.lookup(:my_adapter), my_adapter
         assert_equal ActiveModel::Serializer::Adapter.lookup(:my_subclassed_adapter), my_subclassed_adapter
       ensure