Fix #144 and optimisations

[gavin-lb]

As discussed in the other PR, I'm splitting the optimisations into their own PR.

Since the bug fix is a dependency of the isThreefoldRepetition optimisation, I've included it in this PR.

[jhlywa]

Thanks for the PR @gavin-lb! I'll be mulling over the strict FIDE adherence flags for the next few days.

[ajax333221]

there is an optimization using the half-move clock, you can start counting the positions from newer to older and stop when encounter a 0 in there. Also when move number <= 7 or halfmove <= 7.

      //some code I wrote for another library
      
      clockless_fen = that.getClocklessFenHelper();
      that.fen = clockless_fen + ' ' + that.halfMove + ' ' + that.fullMove;
      that.isThreefold = false;

      if (sliced_fen_history || (that.moveList && that.currentMove > 7 && that.halfMove > 7)) {
        times_found = 1;
        temp = sliced_fen_history || that.fenHistoryExport();
        i = sliced_fen_history ? sliced_fen_history.length - 1 : that.currentMove - 1;

        for (; i >= 0; i--) {
          temp2 = temp[i].split(' ');

          if (temp2.slice(0, 4).join(' ') === clockless_fen) {
            times_found++;

            if (times_found > 2) {
              that.isThreefold = true;
              break;
            }
          }

          if (temp2[4] === '0') {
            break;
          }
        }
      }