Added generic big to 256 method. Implemented new iterator

ethutil/big.go

@@ -39,12 +39,14 @@ func BigD(data []byte) *big.Int {
 // "cast" the big int to a 256 big int (i.e., limit to)
 var tt256 = new(big.Int).Sub(new(big.Int).Lsh(big.NewInt(1), 256), big.NewInt(1))
 
-func To256(x *big.Int) {
+func To256(x *big.Int) *big.Int {
 	x.And(x, tt256)
 
 	if x.Cmp(new(big.Int)) < 0 {
 		x.SetInt64(0)
 	}
+
+	return x
 }
 
 // Big to bytes