contracts: Simplify instantiate_with_code logic

[athei]

Right now, we implement some optimizations when a code is uploaded and instantiation at the same time. That adds a lot of complexity to our code. The right move would be to only support instantiate and upload_code and let users use batch_all if they want to bundle them. It would massively simplify our code.

That said, we can't remove the instantiate_with_code dispatchable for backwards compatibility. But we can refactor it to just emulate batch_all by doing upload_code and instantiate back to back in a storage transaction.