[relax-ng] future TC work

A few things I hope we consider:

- Identity constraints: key/keyref mop up

- Non-XML syntax: an additional committee spec? I think people like it.

- Built-in datatypes: perhaps a few more beyond string and token? int and
float?

- Regular expressions: something natural like this (I don't care what the
<regex> element is called)

   <element name="phone">
    <data type="string">
     <regex>\(\d{3}\)\d{3}-\d{4}</regex>
    </data>
   </element>

-Mike