Allow to output wrapped eliom elements

So, I'm not sure you will want all of this, since the code is meh and it was mainly an experiment to see how that would work with wrapped nodes.

You may at least want to take
 - the correct `with type` invocation, yours was lacking one equality. I think I will add it to tyxml.
 - the mli.

@hhugo : remind me, why didn't we add a module W in Xml_sigs.Wrapped ?! It would basically remove the need for two functors everywhere.