Specify broader lifetime for Children::top() result

[tomekb234]

The function Children::top() in parser/tag.rs returns a reference to RawChildren without a specified lifetime, so the lifetime of &self is implicitly used. This is too restrictive because the reference points to data which is valid within the potentially broader 'b lifetime.

This small patch fixes this problem.

The change allows writing (without unsafe hacks) for example the following convenience function:

fn iter_node_children<'a, 'b>(parser: &'b Parser<'a>, node: &'b Node<'a>) -> impl Iterator<Item = &'b Node<'a>> {
    node.children()
        .map(|children| children.top().iter().map(|handle| handle.get(parser).unwrap()))
        .into_iter()
        .flatten()
}