Impl Default for a few structs

src/data/coarena.rs

@@ -1,7 +1,7 @@
 use crate::data::arena::Index;
 
 #[cfg_attr(feature = "serde-serialize", derive(Serialize, Deserialize))]
-#[derive(Clone, Debug)]
+#[derive(Clone, Debug, Default)]
 /// A container for data associated to item existing into another Arena.
 pub struct Coarena<T> {
     data: Vec<(u32, T)>,

src/data/pubsub.rs

@@ -38,7 +38,7 @@ impl PubSubCursor {
 
 /// A pub-sub queue.
 #[cfg_attr(feature = "serde-serialize", derive(Serialize, Deserialize))]
-#[derive(Clone)]
+#[derive(Clone, Default)]
 pub struct PubSub<T> {
     deleted_messages: u32,
     deleted_offsets: u32,