[racket] Running Racket in the R6RS mode

Unless one really-really *needs* portability to R6RS *right now* (which 
is conceivable but unlikely), I suggest simply going full Racket.

I decided that going full Racket was the most practical for me.  My 
backup plan is that, if I ever do need to go RnRS, I know how to port my 
code.  I now suspect that I'll never need to do a port, since Racket has 
done everything I've needed for years.  I also see a lot of the 
innovation happening in Racket lately, which I think further reduces the 
likelihood of ever wanting to port.

Neil V.