pub fn serialize<K, V, S, T>(
    map: &IndexMap<K, V, S>,
    serializer: T
) -> Result<T::Ok, T::Error>where
    K: Serialize + Hash + Eq,
    V: Serialize,
    S: BuildHasher,
    T: Serializer,Expand description
Serializes an IndexMap as an ordered sequence.
This function may be used in a field attribute for deriving Serialize:
#[derive(Serialize)]
struct Data {
    #[serde(serialize_with = "indexmap::map::serde_seq::serialize")]
    map: IndexMap<i32, u64>,
    // ...
}