Simplify Html5 and Svg signatures using tyxml's signature functors.

See ocsigen/tyxml#50

It doesn't actually fix anything (afaict) but it's simpler (and doesn't make the documentation any worse). It will also auto-update in the future, if the export is changed.

There is one "question" about uri. Currently `uri` is **defined** in Html5_sigs.T as `Xml.uri`, so there is no need to reexport it everywhere (hence why I didn't). I could add an explicit `with type uri = uri` just to make sure that everything is consistent, but it's not necessary (and I doubt we will ever change it).