trang does xsd

This doesn’t have an particular implications ATM, but I JUST realized
that James Clark has somewhere along the way recently added support
for conversion to XML Schema to trang. So, I converted csl.rnc to
csl.xsd, and it validates!

Of course, I have no idea how much looser it needs to be in terms of
validating CSL instances, given that we exploit features in RNG that
XSD does not support.


Ahem … Trang has had this forever; I just forgot.

But I made a few changes to the schema as a result that make the
conversion easier.