Support non-self-describing serde formats

[skeet70]

It seems like non-self-describing formats (ie postcard, bincode) require a sized sequence when serializing. papaya::HashMap obviously can't provide a bounded size_hint (which would make serde's collect_map work as is) due to concurrent access. I've worked around this via

fn serialize_papaya_map<S, K, V>(map: &HashMap<K, V>, serializer: S) -> StdResult<S::Ok, S::Error>
where
    S: Serializer,
    K: Serialize + core::hash::Hash + Eq,
    V: Serialize,
{
    serializer.collect_seq(map.pin().iter().collect::<Vec<_>>())
}
fn deserialize_papaya_map<'de, D, K, V>(deserializer: D) -> StdResult<HashMap<K, V>, D::Error>
where
    D: Deserializer<'de>,
    K: Deserialize<'de> + core::hash::Hash + Eq,
    V: Deserialize<'de>,
{
    let entries: Vec<(K, V)> = Vec::deserialize(deserializer)?;
    let map = HashMap::new();
    for (k, v) in entries {
        map.pin().insert(k, v);
    }
    Ok(map)
}

If you don't want to bring that into the lib, it may be helpful to others to document the incompatibility due to concurrency here and suggest a similar workaround to what I've done. I can PR that if it's the way you'd like to approach it.

[ibraheemdev]

Another option here might be to implement Serialize for &mut HashMap and provide a size hint there, or alternatively provide a FrozenHashMap wrapper, which might be useful in general for other reasons.