Stack-allocated closures

I added a chapter on stack-allocated closures in the ATS-Tutorial book.
It is currently Chapter 27:

http://ats-lang.sourceforge.net/DOCUMENT/ATS2TUTORIAL/HTML/HTMLTOC/
http://ats-lang.sourceforge.net/DOCUMENT/ATS2TUTORIAL/HTML/HTMLTOC/c1249.html

Cheers!

–Hongwei