remove unnecessary bignum conversion

lib/BIP32.js

@@ -281,12 +281,12 @@ BIP32.prototype.deriveChild = function(i) {
   } else {
     var data = Buffer.concat([this.eckey.public, ib]);
     var hash = coinUtil.sha512hmac(data, this.chainCode);
-    var il = bignum.fromBuffer(hash.slice(0, 32), {size: 32});
+    var il = hash.slice(0, 32);
     var ir = hash.slice(32, 64);
 
     // Ki = (IL + kpar)*G = IL*G + Kpar
     var ilGkey = new Key();
-    ilGkey.private = il.toBuffer({size: 32});
+    ilGkey.private = il;
     ilGkey.regenerateSync();
     var ilG = Point.fromKey(ilGkey);
     var oldkey = new Key();