Add methods to xee-xpath that return xot items

[pachi]

All query methods use an XPath string and a function to convert the resulting item to a Rust value. This is convenient for many cases but having a way to directly get the xot node so it can be used to build another XML document, clone a subtree or manipulate it get more cumbersome.

It would be very convenient for further manipulation, being able to use xpath queries that return xot nodes (a node, many nodes, optional nodes or optional vecs of nodes). Something akin to using |_, item| Ok(item.to_node()?) as convert function.

Maybe just having convenience declarations for those functions so they return atoms, nodes or functions could be also an option, so you could do:

q.option("/doc/item", NODE_FN)?

[faassen]

The question here is what to do if the query doesn't return a node but, say, a string? Would you want it to error?