(public) returns index of lowest 1-bit (or -1 if none)
function bnGetLowestSetBit() {
  var this_array = this.array;
  for(var i = 0; i < this.t; ++i)
    if(this_array[i] != 0) return i*BI_DB+lbit(this_array[i]);
  if(this.s < 0) return this.t*BI_DB;
  return -1;
}