Macro encode_params 

macro_rules! encode_params {
    ($w:expr, $version:expr, $($key:expr => $val:expr),* $(,)?) => { ... };
}

Encode message parameters with compile-time sorted keys.

Keys must be listed in ascending order (enforced at compile time). Option<T> values are skipped when None.

encode_params!(w, version,
    0x10 => self.forward,
    0x20 => self.subscriber_priority,
);