[plt-scheme] R6RS Interoperability

> Unfortunately, as far as we can tell no one has ported SSAX to R6RS yet,
> so we may have to tackle this task ourselves.

http://github.com/GreyLensman/rl3/tree/0a6e8a405cf4caa783df96f55d22cef671346d32/rl3/xml

http://github.com/GreyLensman/rl3/commits/webserver

It is not comprehensive but basic SXML, SXPATH and HTMLPRAG work.

Partial changes include.  '@' is not a valid leading char in R6RS (R5RS
for that matter I think), therefore a '*' is used instead for
attributes.

e.g. (p (@ (id "p1")) => (p (* (id "p1"))