mv of babyjub.PublicKey to babyjub.PublicKeyComp

Update usage of `*babyjub.PublicKey` to `babyjub.PublicKeyComp`
- when the key is not defined, internally is used `babyjub.EmptyBJJComp`, which is a `[32]byte` of zeroes of type `babyjub.PublicKeyComp`
- the API continues returning `nil` when the key is not defined

common/zk_test.go

@@ -11,6 +11,6 @@ import (
 func TestZKInputs(t *testing.T) {
 	zki := NewZKInputs(100, 16, 512, 24, 32, big.NewInt(1))
 	_, err := json.Marshal(zki)
-	require.Nil(t, err)
+	require.NoError(t, err)
 	// fmt.Println(string(s))
 }