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_test.rb

@@ -6,7 +6,7 @@ module ActiveModel
       def setup
         profile = Profile.new
         @serializer = ProfileSerializer.new(profile)
-        @adapter = ActiveModel::Serializer::Adapter.new(@serializer)
+        @adapter = ActiveModel::Serializer::Adapter::Base.new(@serializer)
       end
 
       def test_serializable_hash_is_abstract_method