further simplify use of bignum in Transaction and ScriptInterpreter

lib/ScriptInterpreter.js

@@ -494,14 +494,14 @@ ScriptInterpreter.prototype.eval = function eval(script, tx, inIndex, hashType,
                 break;
 
               case Opcode.map.OP_LSHIFT:
-                if (v2.cmp(0) < 0 || v2.cmp(bignum(2048)) > 0) {
+                if (v2.cmp(0) < 0 || v2.cmp(2048) > 0) {
                   throw new Error("OP_LSHIFT parameter out of bounds");
                 }
                 num = v1.shiftLeft(v2);
                 break;
 
               case Opcode.map.OP_RSHIFT:
-                if (v2.cmp(0) < 0 || v2.cmp(bignum(2048)) > 0) {
+                if (v2.cmp(0) < 0 || v2.cmp(2048) > 0) {
                   throw new Error("OP_RSHIFT parameter out of bounds");
                 }
                 num = v1.shiftRight(v2);

lib/Transaction.js

@@ -512,7 +512,7 @@ Transaction.prototype.fromObj = function fromObj(obj) {
     var addr = new Address(addrStr);
     var script = Script.createPubKeyHashOut(addr.payload());
 
-    var valueNum = new bignum(obj.outputs[addrStr]);
+    var valueNum = bignum(obj.outputs[addrStr]);
     var value = util.bigIntToValue(valueNum);
 
     var txout = new TransactionOut();