[CBOR] Lotus and go-f3 ser/de compatibility.

[ruseinov]

While investigating go-f3 and lotus implementations I have stumbled upon the generated code that checks certain fields for length. That's specified as e.g. cborgen:"maxlen=1000000000.

Some examples:

1. Lotus
2. Go-F3

Going forward it might be nice to have sort of library that'd wrap cbor ser/de and be able to read attributes.
Another thing is that for structs we currently need to implement custom ser/de as follows:

impl Serialize for PowerEntry {
    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
    where
        S: serde::Serializer,
    {
        if !self.pub_key.is_valid() {
            return Err(serde::ser::Error::custom(
                "Byte array in field pub_key is too long",
            ));
        }


        (&self.id,
         &self.power,
         &self.pub_key).serialize(serializer)
    }
}

impl<'de> Deserialize<'de> for PowerEntry {
    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
    where
        D: serde::Deserializer<'de>,
    {
        let (id, power, pub_key) = Deserialize::deserialize(deserializer)?;
        Ok(Self {
            id,
            power,
            pub_key,
        })
    }
}

What this does:

1. Implements said length constraints.
2. Converts a struct into a tuple in order to serialize
3. Converts a tuple into a struct in order to deserialize.

If those could be automated - that'd save a lot of boilerplate. Field ordering is easy - just serialize in that same order they are defined. If any of the struct fields need to be transient - have an attribute to ignore them for cbor ser/de.