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