(public) return value as integer
function bnIntValue() {
  var this_array = this.array;
  if(this.s < 0) {
    if(this.t == 1) return this_array[0]-BI_DV;
    else if(this.t == 0) return -1;
  }
  else if(this.t == 1) return this_array[0];
  else if(this.t == 0) return 0;
  // assumes 16 < DB < 32
  return ((this_array[1]&((1<<(32-BI_DB))-1))<<BI_DB)|this_array[0];
}