Converts a RuleNode parse tree to a lark Tree.

def _to_tree(self, rule_node):
        """Converts a RuleNode parse tree to a lark Tree."""
        orig_rule = self.orig_rules[rule_node.rule.alias]
        children = []
        for child in rule_node.children:
            if isinstance(child, RuleNode):
                children.append(self._to_tree(child))
            else:
                assert isinstance(child.name, Token)
                children.append(child.name)
        t = Tree(orig_rule.origin, children)
        t.rule=orig_rule
        return t