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7adbf9c6af
As the final step in removing `pnet_packet`, we need to introduce `-Mut` equivalent slices for UDP, TCP and ICMP packets. As a starting point, introducing `UpdHeaderSliceMut` and `TcpHeaderSliceMut` is fairly trivial. The ICMP variants are a bit trickier because those are different for IPv4 and IPv6. Additionally, ICMP for IPv4 is quite complex because it can have a variable header length. Additionally. for both variants, the values in byte range 5-8 are semantically different depending on the ICMP code. This requires us to design an API that balances ergonomics and correctness. Technically, an ICMP identifier and sequence can only be set if the ICMP code is "echo request" or "echo reply". However, adding an additional parsing step to guarantee this in the type system is quite verbose. The trade-off implemented in this PR allows to us to directly write to the byte 5-8 using the `set_identifier` and `set_sequence` functions. To catch errors early, this functions have debug-assertions built in that ensure that the packet is indeed an ICMP echo packet. Resolves: #6366.