New NaN-related and min/max flonum procedures and fixes.

New NaN-related and min/max flonum procedures and fixes.

- (flo:qnan) returns a quiet NaN: arithmetic on it quietly returns
  another qNaN (usually the same one, unless there are multiple to
  choose from) without raising any exception.

- (flo:snan) returns a signalling NaN: arithmetic on it returns
  another sNaN (usually the same) and raises an exception, which will
  trap if you ask.

- (flo:qnan? f) = (and (flo:nan? f) (flo:nan-quiet? f))

- (flo:snan? f) = (and (flo:nan? f) (not (flo:nan-quiet? f)))

- (flo:min x y) and (flo:max x y) now raise an exception only if at
  least one input is a signalling NaN, and traps only if you ask.
  These now implement minNum and maxNum of IEEE 754-2008.

- (flo:min-mag x y) and (flo:max-mag x y) return whichever of the
  inputs has the smaller magnitude, as in minNumMag and maxNumMag in
  IEEE 754-2008.