Function world__retrieve_messages 

pub unsafe extern "C" fn world__retrieve_messages(
    message_struct_size: u32,
    entities_ptr: *const EntityHandle,
    entities_len: u32,
    out_msgs_ptr: *mut Message,
    out_msgs_len: u32,
)

message_struct_size is the size of the module struct to sanity check size and have entities is a list of entities for which to retrieve messages. out_msgs will contain the messages of the outbox of each entity in entities after a call to this function. Use get_message_counts to retrieve the total number of msgs to allocate the right amount of space.