[R6RS] R5RS-compliant mode

[William D Clinger]

Marc has responded to my question by private email.  Thank you, Marc.

Perhaps a partial list of the portability problems I am encountering
will encourage others to respond:

  * missing features, e.g. the R5RS macro system
  * case-sensitivity
  * nonstandard escape characters (e.g. vertical bars) being printed
    for symbols of mixed case that are created by STRING->SYMBOL
  * nonstandard printing of vectors, e.g. #3(a b c) instead of #(a b c)
  * nonstandard printing of lists, e.g. square brackets instead of
    parens when printing lists that look like Scheme code

In case you're wondering why I need this degree of compatibility,
it's because I'm using scanner and parser generators that output
code in several different languages, including Java and Scheme,
and because I am allowing my students to use any standards-compliant
implementation of these languages.  If the implementations aren't
standards-compliant, it becomes well-nigh impossible for my students
to put together a compiler from scanners, parsers, and other modules
that were generated or written in different implementations by
different students.  It's a good practical test of portability.

Will