simplify use of bignum in Base58 and Block

lib/Base58.js

@@ -24,9 +24,9 @@ var base58 = {
       str = new Buffer(buf.length << 1);
     }
     var i = str.length - 1;
-    while (x.gt(new bignum(0))) {
-      r = x.mod(new bignum(58));
-      x = x.div(new bignum(58));
+    while (x.gt(0)) {
+      r = x.mod(58);
+      x = x.div(58);
       str[i] = ALPHABET_BUF[r.toNumber()];
       i--;
     }
@@ -44,10 +44,10 @@ var base58 = {
 
   decode: function(str) {
     if (str.length == 0) return zerobuf;
-    var answer = new bignum(0);
+    var answer = bignum(0);
     for (var i = 0; i < str.length; i++) {
-      answer = answer.mul(new bignum(58));
-      answer = answer.add(new bignum(ALPHABET_INV[str[i]]));
+      answer = answer.mul(58);
+      answer = answer.add(ALPHABET_INV[str[i]]);
     };
     var i = 0;
     while (i < str.length && str[i] == ALPHABET_ZERO) {

lib/Block.js

@@ -114,7 +114,7 @@ Block.prototype.checkProofOfWork = function checkProofOfWork() {
  */
 Block.prototype.getWork = function getWork() {
   var target = util.decodeDiffBits(this.bits, true);
-  return BlockRules.largestHash.div(target.add(new Bignum(1)));
+  return BlockRules.largestHash.div(target.add(1));
 };
 
 Block.prototype.checkTimestamp = function checkTimestamp() {