Purify Categories

Apply the same methodology, as in Revision 432,
to the category chunk-handler. This completes
the replacement of all the code that looks like

  if string.is_utf8?
    do something
  else
    complain
  end

with code that looks like

  string.purify
  do something

lib/stringsupport.rb

@@ -31,7 +31,7 @@ class String
        |  \xF0[\x90-\xBF][\x80-\xBF]{2}     # planes 1-3
        | [\xF1-\xF3][\x80-\xBF]{3}          # planes 4-15
        |  \xF4[\x80-\x8F][\x80-\xBF]{2}     # plane 16
-     )+\Z/nx;
+     )*\Z/nx;
 #++
      
 # Check whether a string is valid utf-8