[gambit-list] First fix to "New _num.scm"

The new fft routine is a complex fft rather than a real fft, and so a bigger intermediate f64 array is used for a multiplication of a bigger size, so we need

(define ##bignum.fft-mul-max-width (if (##fixnum? -1073741824) ; to avoid creating f64vectors that are too long 536870912 2097152)) (set! ##bignum.fft-mul-max-width ##bignum.fft-mul-max-width)

instead of

(define ##bignum.fft-mul-max-width (if (##fixnum? -1073741824) ; to avoid creating f64vectors that are too long 536870912 4194304)) (set! ##bignum.fft-mul-max-width ##bignum.fft-mul-max-width)

Brad