[1.x] Replace write_to_storage with export_state_update

[tomleavy]

Background

Part of #211

Before (0.x)

// Make a group state store that conforms to the GroupStateStorage trait
let group_state_store = MyGroupStateStore::new();

let client = Client::builder()
    .group_state_storage(group_state_store) // Transfer the ownership of the group state storage
    ....
    .build();
        
// ..... load and use a chat

chat.write_to_storage().unwrap();

In the above, write_to_storage first calls GroupStateStorage::write with the current GroupState along along with a set of updates to prior epoch data. Updates to prior epoch data include either overwriting secret tree data used for message decryption or completely deleting cached epoch data to lock in forward secrecy.

After (1.x)

Export State Update API

Instead of calling various storage interfaces internally, the export_state_update function will output a full copy of a Group's public and secret information, along with any prior epoch data that has been modified since the last time export_state_update has been called.

Calling export_state_update clears all prior epoch data in order to reduce memory footprint within mls-rs

// New write to storage interface
pub fn export_state_update() -> Result<GroupStateUpdate, MlsError>;

// GroupStateUpdate definition
pub struct GroupStateUpdate {
    pub state: GroupState, // Should expose the properties of internal Snapshot struct in a sane way
    pub past_epoch_data: Vec<EpochRecord>, // Should expose non-secret values of PriorEpoch in some way
}