`\sqrt{xy}` does not need an inner `<mrow>`

[tmke8]

Changing the definition to

    Sqrt(&'arena [&'arena Node<'arena>]),

is easy enough and the emitting code is also straight-forward:

            Node::Sqrt(content) => {
                write!(s, "<msqrt>")?;
                for node in *content {
                    node.emit(s, child_indent)?;
                }
                writeln_indent!(s, base_indent, "</msqrt>");
            }

but how should this be handled in the parser? We currently call self.parse_token(next, ParseAs::Arg, Class::Default) there, which returns a Node and not a list of nodes.