Macros for ICMP and ICMPv6 payloads

[jwhb]

This MR adds macros to use payload instructions for ICMP and ICMPv6.

This allows to do:

/// IPv4
rule.add_expr(&nft_expr!(payload icmp icmptype));
rule.add_expr(&nft_expr!(cmp == 8_u8));

rule.add_expr(&nft_expr!(payload icmp code));
rule.add_expr(&nft_expr!(cmp == 7_u8));

/// IPv6
rule.add_expr(&nft_expr!(payload icmpv6 icmptype));
rule.add_expr(&nft_expr!(cmp == 133_u8));

rule.add_expr(&nft_expr!(payload icmpv6 code));
rule.add_expr(&nft_expr!(cmp == 129_u8));

For ICMP type the keyword is icmptype. The keyword type can not be used, as it is a reserved keyword in Rust.

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