Associations refactoring

* Move all associations related code from Serializer class to Associations module
* Introduce Reflection class hierarchy
* Introduce Association class
* Rid off Serializer#each_association
* Introduce Serializer#associations enumerator

lib/active_model/serializer/belongs_to_reflection.rb

@@ -0,0 +1,10 @@
+module ActiveModel
+  class Serializer
+    # @api private
+    class BelongsToReflection < SingularReflection
+      def macro
+        :belongs_to
+      end
+    end
+  end
+end