Handle conflicts between key names and serializer methods

As an example, all serializers implement `#object` as a reference to the
object being esrialized, but this was preventing adding a key to the
serialized representation with the `object` name.

Instead of having attributes directly map to methods on the serializer,
we introduce one layer of abstraction: the `_attributes_map`. This hash
maps the key names expected in the output to the names of the
implementing methods.

This simplifies some things (removing the need to maintain both
`_attributes` and `_attribute_keys`), but does add some complexity in
order to support overriding attributes by defining methods on the
serializer. It seems that with the addition of the inline-block format,
we may want to remove the usage of programatically defining methods on
the serializer for this kind of customization.

test/serializers/attribute_test.rb

@@ -43,6 +43,15 @@ module ActiveModel
         assert_equal({ blog: { id: 'AMS Hints' } }, adapter.serializable_hash)
       end
 
+      def test_object_attribute_override
+        serializer = Class.new(ActiveModel::Serializer) do
+          attribute :name, key: :object
+        end
+
+        adapter = ActiveModel::Serializer::Adapter::Json.new(serializer.new(@blog))
+        assert_equal({ blog: { object: 'AMS Hints' } }, adapter.serializable_hash)
+      end
+
       def test_type_attribute
         attribute_serializer = Class.new(ActiveModel::Serializer) do
           attribute :id, key: :type