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

api/accountcreationauths.go

@@ -61,7 +61,7 @@ type receivedAuth struct {
 func accountCreationAuthAPIToCommon(apiAuth *receivedAuth) *common.AccountCreationAuth {
 	return &common.AccountCreationAuth{
 		EthAddr:   ethCommon.Address(apiAuth.EthAddr),
-		BJJ:       (*babyjub.PublicKey)(&apiAuth.BJJ),
+		BJJ:       (babyjub.PublicKeyComp)(apiAuth.BJJ),
 		Signature: []byte(apiAuth.Signature),
 		Timestamp: apiAuth.Timestamp,
 	}