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

test/l2db.go

@@ -78,7 +78,7 @@ func GenAuths(nAuths int) []*common.AccountCreationAuth {
 		// Generate auth
 		auth := &common.AccountCreationAuth{
 			EthAddr: ethCrypto.PubkeyToAddress(ethPrivK.PublicKey),
-			BJJ:     bjjPrivK.Public(),
+			BJJ:     bjjPrivK.Public().Compress(),
 		}
 		// Sign
 		h, err := auth.HashToSign()