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

test/serializers/association_macros_test.rb

@@ -0,0 +1,36 @@
+require 'test_helper'
+
+module ActiveModel
+  class Serializer
+    class AssociationMacrosTest < Minitest::Test
+      AuthorSummarySerializer = Class.new
+      class AssociationsTestSerializer < Serializer
+        belongs_to :author, serializer: AuthorSummarySerializer
+        has_many :comments, embed: :ids
+        has_one :category
+      end
+
+      def before_setup
+        @reflections = AssociationsTestSerializer._reflections
+      end
+
+      def test_has_one_defines_reflection
+        has_one_reflection = HasOneReflection.new(:category, {})
+
+        assert_includes(@reflections, has_one_reflection)
+      end
+
+      def test_has_many_defines_reflection
+        has_many_reflection = HasManyReflection.new(:comments, embed: :ids)
+
+        assert_includes(@reflections, has_many_reflection)
+      end
+
+      def test_belongs_to_defines_reflection
+        belongs_to_reflection = BelongsToReflection.new(:author, serializer: AuthorSummarySerializer)
+
+        assert_includes(@reflections, belongs_to_reflection)
+      end
+    end
+  end
+end