Improve .toString(2) performance

Author: Azero123

BigInteger.js

@@ -1314,6 +1314,7 @@ var bigInt = (function (undefined) {
 
     BigInteger.prototype.toString = function (radix, alphabet) {
         if (radix === undefined) radix = 10;
+        if (radix === 2) return (this.sign ? "-" : "") + this.toArray(2).value.join('');
         if (radix !== 10) return toBaseString(this, radix, alphabet);
         var v = this.value, l = v.length, str = String(v[--l]), zeros = "0000000", digit;
         while (--l >= 0) {