(public) convert to bigendian byte array
function bnToByteArray() {
  var this_array = this.array;
  var i = this.t, r = new Array();
  r[0] = this.s;
  var p = BI_DB-(i*BI_DB)%8, d, k = 0;
  if(i-- > 0) {
    if(p < BI_DB && (d = this_array[i]>>p) != (this.s&BI_DM)>>p)
      r[k++] = d|(this.s<<(BI_DB-p));
    while(i >= 0) {
      if(p < 8) {
        d = (this_array[i]&((1<<p)-1))<<(8-p);
        d |= this_array[--i]>>(p+=BI_DB-8);
      }
      else {
        d = (this_array[i]>>(p-=8))&0xff;
        if(p <= 0) { p += BI_DB; --i; }
      }
      if((d&0x80) != 0) d |= -256;
      if(k == 0 && (this.s&0x80) != (d&0x80)) ++k;
      if(k > 0 || d != this.s) r[k++] = d;
    }
  }
  return r;
}