Function build_echo_reply 

pub fn build_echo_reply(
    src: &Ipv6Address,
    dst: &Ipv6Address,
    id: u16,
    seq: u16,
    data: &[u8],
) -> Vec<u8>

Build an ICMPv6 Echo Reply message.

Returns the raw ICMPv6 message bytes (not wrapped in IPv6 header).