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/base.rb

@@ -0,0 +1,58 @@
+module ActiveModel
+  class Serializer
+    module Adapter
+      class Base
+        # Automatically register adapters when subclassing
+        def self.inherited(subclass)
+          ActiveModel::Serializer::Adapter.register(subclass)
+        end
+
+        attr_reader :serializer, :instance_options
+
+        def initialize(serializer, options = {})
+          @serializer = serializer
+          @instance_options = options
+        end
+
+        def serializable_hash(_options = nil)
+          fail NotImplementedError, 'This is an abstract method. Should be implemented at the concrete adapter.'
+        end
+
+        def as_json(options = nil)
+          hash = serializable_hash(options)
+          include_meta(hash)
+          hash
+        end
+
+        def fragment_cache(*_args)
+          fail NotImplementedError, 'This is an abstract method. Should be implemented at the concrete adapter.'
+        end
+
+        def cache_check(serializer)
+          CachedSerializer.new(serializer).cache_check(self) do
+            yield
+          end
+        end
+
+        private
+
+        def meta
+          serializer.meta if serializer.respond_to?(:meta)
+        end
+
+        def meta_key
+          serializer.meta_key || 'meta'.freeze
+        end
+
+        def root
+          serializer.json_key.to_sym if serializer.json_key
+        end
+
+        def include_meta(json)
+          json[meta_key] = meta if meta
+          json
+        end
+      end
+    end
+  end
+end