`preceding::` needs to yield all preceding nodes, excluding ancestors.
`following::` needs to yield all following nodes, excluding descendants.
Both `Node::preceding_nodes` and `Node::following_nodes` (which we're
currently using) don't quite match these requirements, so we have to
engineer some xpath-specific iterators.
Testing: New tests start to pass
---------
Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>
5ecf09d76a