Generic ICMPv4 packet (RFC 792).
The 4 bytes RFC 792 calls "rest of header" are part of payload here;
their meaning is type-specific ((identifier, sequence) for Echo,
(unused, nextHopMtu) for some Destination Unreachable codes, etc.).
code: number
ICMP code field, type-specific subtype.
checksum: number
Big-endian 16-bit Internet checksum over the entire ICMP message
(header + payload) with this field set to zero. Not auto-computed on
encode; use internetChecksum from @binstruct/inet.
payload: Uint8Array<ArrayBufferLike>
Remaining bytes of the message.