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

CHANGELOG.md

@@ -1,5 +1,15 @@
 ### 0.10.0
 
+Breaking changes:
+  * Adapters now inherit Adapter::Base. 'Adapter' is now a module, no longer a class. [@bf4], #1138
+    * 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
+
+Features:
   * adds adapters pattern
   * adds support for `meta` and `meta_key` [@kurko]
   * adds method to override association [@kurko]
@@ -12,3 +22,7 @@
   * adds FlattenJSON as default adapter [@joaomdmoura]
   * adds support for `pagination links` at top level of JsonApi adapter [@bacarini]
   * adds extended format for `include` option to JsonApi adapter [@beauby]
+
+Fixes:
+
+Misc: