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/debugapi/debugapi_test.go

@@ -35,7 +35,7 @@ func newAccount(t *testing.T, i int) *common.Account {
 		TokenID:   common.TokenID(i),
 		Nonce:     common.Nonce(i),
 		Balance:   big.NewInt(1000),
-		PublicKey: pk,
+		PublicKey: pk.Compress(),
 		EthAddr:   address,
 	}
 }