Function serialize
pub fn serialize<A, S>(
    a: &Option<A>,
    s: S,
) -> Result<<S as Serializer>::Ok, <S as Serializer>::Error>where
    A: SerdeAmountForOpt,
    S: Serializer,pub fn serialize<A, S>(
    a: &Option<A>,
    s: S,
) -> Result<<S as Serializer>::Ok, <S as Serializer>::Error>where
    A: SerdeAmountForOpt,
    S: Serializer,