Aggregate Commands may need Deserialize trait

[tavakyan]

Sending events over channels is possible because DomainEvents can be serialized and deserialized before sending and after receiving.

However, in cases where a command is to be sent, a channel sending and receiving aggregate commands cannot generally be constructed and has to be handled on a case by case basis for each aggregate command type.

One way to fix this is to add a constraint to the Aggregate trait such as:

#[async_trait]
pub trait Aggregate: Default + Serialize + DeserializeOwned + Sync + Send {
    /// Specifies the inbound command used to make changes in the state of the Aggregate.
    type Command: Serialize + DeserializeOwned + fmt::Debug + Sync + Send;
...
}

See #32

[tavakyan]

Actually going to close this because I was missing a simple where clause in the traits I was designing... Just had to do something like

#[async_trait]
pub trait CommandForwarder<A: Aggregate>: Send + Sync {
    async fn forward(&self, cmd: A::Command)
    where
        <A as Aggregate>::Command: Serialize;
}