PacketFoobar::packet_size(_packet: &Foobar) is broken

packet_size is supposed to return "The size (in bytes) of an instance when converted into a byte-array".

However, this doesn't work correctly when complex variable length fields are present.

Consider the following test:

#[test]
fn test_tcp_packet_size() {
    let tcp = Tcp{
        source: 36328,
        destination: 443,
        sequence: 2012456774,
        acknowledgement: 825842053,
        data_offset: 8,
        reserved: 0,
        flags: 24,
        window: 32,
        checksum: 39435,
        urgent_ptr: 0,
        options: vec![
            TcpOption::nop(),
            TcpOption::nop(),
            TcpOption::timestamp(0xdeadbeef, 0xdeadbeef),
        ],
        payload: vec![],
    };

    // fixed fields + nop + nop + timestamp
    let expected_length = 20 + 1 + 1 + 10;

    assert_eq!((tcp.data_offset as usize) * 4 + tcp.payload.len(), expected_length);
    assert_eq!(MutableTcpPacket::packet_size(&tcp), expected_length);
}

The test will fail:

thread 'tcp::test_tcp_packet_size' panicked at 'assertion failed: `(left == right)`
  left: `23`,
 right: `32`', src/tcp.rs:464:5

I took a look at how these macros were expanded:

/// The size (in bytes) of a Tcp instance when converted into
/// a byte-array
#[inline]
pub fn packet_size(_packet: &Tcp) -> usize {
    20 + _packet.options.len() + _packet.payload.len()
}

Instead of calculating actual size in bytes, it just returns a number of TCP options, which is wrong.

Start with the failing test and generated packet_size function shown in the issue, using src/tcp.rs:464 as the failure location. Trace how the packet macros calculate sizes for variable-length TCP options, then run the TCP packet-size test and confirm that the timestamp option contributes its byte length rather than only the option count.