[R6RS] R5RS-compliant mode

[Marc Feeley]

> Thank you, Marc.  I also need to know how to turn off the vertical
> bars that are printed around symbols of mixed case that are created
> by STRING->SYMBOL.

In version 3 of Gambit you can't turn off the vertical bars.  But as
far as I can tell this is R5RS compliant.  In fact, in the "rationale"
of write and display there is:

  "Implementations that allow "slashification" within symbols will
  probably want write but not display to slashify funny characters."

So I think Gambit's behaviour is closer to the spirit of R5RS than not
using the vertical bars.  The important thing is that the syntax used
by write is understood by read.

Note that Gambit 4 does something even more unconventional but
R5RS compliant with "display": lists and vectors are displayed without
the parentheses and spaces (and # for vectors).  This means that
the following code:

 (let ((a 5) (b 10) (c 3))
   (display (list "I have "
                  (cons a " apples, ")
                  (vector b " bananas and " c " chocolates.\n"))))

displays

  I have 5 apples, 10 bananas and 3 chocolates.

This feature is useful for formatted output.

Marc