Return a new free identifier as :class:`~jinja2.nodes.InternalName`.

def free_identifier(self, lineno=None):
        """Return a new free identifier as :class:`~jinja2.nodes.InternalName`."""
        self._last_identifier += 1
        rv = object.__new__(nodes.InternalName)
        nodes.Node.__init__(rv, 'fi%d' % self._last_identifier, lineno=lineno)
        return rv