Function register_aggregate 

pub fn register_aggregate<A, D, T>(
    connection: &Connection,
    options: FunctionOptions,
    aggregate: D,
) -> Result<()>
where
    A: RefUnwindSafe + UnwindSafe,
    D: Aggregate<A, T> + 'static,
    T: SqlFnOutput,

Register a stateful aggregate callback on one SQLite connection.