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.
