[plt-scheme] Raw HTML in web server output

FYI:

I just got back to this problem this morning after spending yesterday  
messing around in the whacky world of thesis submission.

In the end I adopted a fairly heavy-handed approach. I re-implemented  
write-shtml-as-html and changed the way foreign filters work: rather  
than getting the filter to return XSHTML, my version passes it the  
output stream and let it write its output directly.

The filter is also given enough information that, if it wants to, it  
can build an XSHTML fragment and pass it back into the main write  
procedure for the same functionality as Neil's procedure.

This involved a fair amount of redundant coding and it's not very  
elegant, but it seems to work okay and it allows me to embed little  
blocks of malformed XHTML in the page without sacrificing the  
convenience of SXHTML's tree structures.

Cheers,

-- Dave