(protected) this += n << w words, this >= 0
function bnpDAddOffset(n,w) {
  var this_array = this.array;
  while(this.t <= w) this_array[this.t++] = 0;
  this_array[w] += n;
  while(this_array[w] >= BI_DV) {
    this_array[w] -= BI_DV;
    if(++w >= this.t) this_array[this.t++] = 0;
    ++this_array[w];
  }
}