New backquote.

[MACRAK at MIT-MC (Stavros M. Macrakis)]

The idea is nice, but I woul prefer that the internal names be a little
nicer if we're going to see so much of them.  Even un-grinef'd code
should be marginally readable.  I suggest something like
(|`-expander| (|`,| . B) (|`,@| . D)).  These names shouldn't be too
much in demand.

By the way, there is a bug with readmacro inverse which screws up
on forms like (quote . a); this is not an unreasonable form to have
around: consider '((cond . docond) (quote . doquote)) , for example.
One curious anomaly is that '(quote . x) prints out as ''#232342
but (quote . y) gives an error message saying that y isn't a list
(arg to length); this presumably has something to do with the 
predictor?