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/ethclient_test.go

@@ -158,7 +158,7 @@ func TestClientRollup(t *testing.T) {
 		tx := common.L1Tx{
 			FromIdx:       0,
 			FromEthAddr:   keys[i].Addr,
-			FromBJJ:       keys[i].BJJPublicKey,
+			FromBJJ:       keys[i].BJJPublicKey.Compress(),
 			TokenID:       common.TokenID(0),
 			Amount:        big.NewInt(0),
 			DepositAmount: big.NewInt(10 + int64(i)),