[plt-scheme] xexprs and sxml

I'm all in favor of supporting better XML generation facilities.  But
I am not in favor of changing the Web server's defaults.  We have a
bit of a tendency to change things around, a luxury we can't really
afford in a language without a static type system.  PLT s-expressions,
while not very strong, are surprisingly useful for basic tasks; and
with the growing use of tools like CSS, you don't need a whole lot
more for *basic* Web programming.

The Web server already has the ability to serve different file types
besides XML expressed as x-expressions.  I'm happy to see extensions
that enable WebIt! and SXML.  But I want us to stay backwardly
compatible.

Shriram