fast_xs, a fast C extension for cleaning XML

Overview

The original fast_xs method is based on the xchar code by Sam Ruby:

  http://intertwingly.net/stories/2005/09/28/xchar.rb
  http://intertwingly.net/blog/2005/09/28/XML-Cleansing

_why also packages a slightly modified version in the latest version
of Hpricot (not-yet-released) that also escapes '"' to """ (which
is optional).

This is an exact translation (to the best of my knowledge :) of Sam's
original implementation, so it does not escape """.  The version
of XML::Builder packaged with Rails also uses Sam's version, and
XML::Builder as packaged in Rails 2.0 will be automatically use
fast_xs if available.