Function encode_map_request 

pub fn encode_map_request(
    tcp: bool,
    internal_port: u16,
    suggested_external_port: u16,
    lifetime_secs: u32,
) -> [u8; 12]

Encode a NAT-PMP map-port request (RFC 6886 §3.3): [version][opcode][reserved:2][internal_port:2][suggested_external_port:2][lifetime_secs:4].