Can't use query arguments inside transactions

(maybe this is really gel-rust)

```
_localdev:main> start transaction;
OK: START TRANSACTION
_localdev:main[tx]> select <str>$0;
Parameter <str>$0: aaaa
gel error: TransactionError: current transaction is aborted, commands ignored until end of transaction block
```

Maybe the problem is that we always try an `execute` first instead of `parse`, and that produces a real error...