Improve database type safety

The database is now bound to a type eg. alice::State or bob::State.
The caller cannot expect to retrieve a type that is different to
the type that was stored.

xmr-btc/src/bob.rs

@@ -82,7 +82,7 @@ pub async fn next_state<
     }
 }
 
-#[derive(Debug)]
+#[derive(Debug, Deserialize, Serialize)]
 pub enum State {
     State0(State0),
     State1(State1),