Fixed #13252 -- Serialise with natural keys instead of primary keys.

Add ``--natural-foreign`` and ``--natural-primary`` options, and
deprecate the ``--natural`` option to the ``dumpdata`` management
command.

Add ``use_natural_foreign_keys`` and ``use_natural_primary_keys``
arguments, and deprecate the ``use_natural_keys`` argument to
``django.core.serializers.Serializer.serialize()``.