"Simpler" bigfloat and complex bigfloat arithmetic

There are lots of possible classes.  It is easy to start programming up

real-interval:to
quad-float:to
polar-double-float:to
projective-rational:to    [includes infinity]
affine-rational:to        [includes infinity]
...

and then maxima:to
  must be extended also

That's what I tried to systematize with the lisp generic package. I don't
know that I succeeded in making it as simple as possible.

I think that bigfloat / complex bigfloat is probably the most useful for 
maxima.

RJF