Simplify Tyxml_js signatures using tyxml's signature functors.

Export correctly wrap equalities.
Do not erase Html5.Xml and Svg.Xml (they are needed by the syntax extension).
Use explicitly tot and toelt for coercions with Xml.elt.
Merge pull request #192 from ocsigen/simple_sig

Simplify Tyxml_js signatures and fix usage of tyxml's syntax extension.