Make the PacketPileParser API safe
Currently, the PacketPileParser exposes the PacketParser for interacting with the currently parsed packet. But, the documentation warns about that the exposed PacketParser must not be advanced using .next() or .recurse().
It would be nice to enforce this in code. Maybe our RustFoo allows us this now.
Alternatively, we could decide that the PacketPileParser is not that useful after all, and remove it.