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

lib/active_model/serializer/adapter/null.rb

@@ -1,7 +1,7 @@
 module ActiveModel
   class Serializer
-    class Adapter
-      class Null < Adapter
+    module Adapter
+      class Null < Base
         def serializable_hash(options = nil)
           {}
         end