a :clear option for rlet

[engber@aristotle.ils.nwu.edu (Mike Engber)]

It would be nice if rlet allowed a :clear option like make-record
does. This would be especially helpful for allocating parameter
blocks which are notorious for causing problems unless 'unused'
fields are zeroed out.

I realize there is probably not as nice(efficient) a way to do
this as there is for make-record, but it would still be helpful.
I much prefer rlet to make-record.

If someone can tell me a slick way to zero the contents of a
macptr I'd be happy to implement it myself. Off the top of
my head all I can think of is a loop and %put-xxx. There has
to be a better way.

-ME