Fix code generation for command overflow hooks

[bocchino]

To call the hooks in the way they are currently designed, we will need to deserialize the command arguments before calling the hook. See #495. This will require some refactoring of the code gen, as we will need to do the same thing as in the code that calls the command handlers.

Alternatively, we could do what the command pre-message hooks currently do, and just pass the opcode and command sequence number, without the arguments.

This issue is blocked on #493 to avoid conflicts.

[bocchino]

Unblocking this issue. We'll do it the simple way, which shouldn't cause undue conflicts.