[plt-scheme] HtmlPrag doesn't yet work with PLT 4.0

Some people are noticing that HtmlPrag doesn't work with PLT 4.0.

A solution might take a while, since I need to find time to rewrite one 
of the key algorithms in addition to other changes.  (HtmlPrag was the 
first Scheme code I wrote, and I used mutable pairs heavily.)  I already 
tried a stopgap kludge, but it was not satisfactory.

If anyone's dissertation is hanging in the balance, let me know, and I 
can prioritize this ahead of some other things.