[1.x] Parse message function

[mulmarta]

Background

Part of #211

Parse Message

We will have a function that parses an MLSMessage and outputs information needed to fetch the right data from storage.

pub enum MlsMessageDescription<'a> {
    Welcome {
        key_package_refs: &'a [KeyPackageRef],
        cipher_suite: CipherSuite,
    },
    ProtocolMessage {
        group_id: &'a [u8],
        epoch_id: u64,
        content_type: ContentType, // commit, proposal, or application
    },
    // Processing GroupInfo and KeyPackage does not require any storage
    GroupInfo,
    KeyPackage,
}

impl<'a> From<&'a MlsMessage> for MlsMessageDescription<'a> {
    ...
}

impl MlsMessage {
    fn description(&self) -> MlsMessageDescription  {
        ....
    }
}

[CaioSym]

Seems odd that we would need an object to own the conversion function. Would this have any major problems?

impl From<&MlsMessage> for ParsedMessage {
...
}

[mulmarta]

Yes that's much better.