Add default implementations for StateEvent.

src/core.rs

@@ -21,7 +21,12 @@ pub trait RoomEvent<'a, T>: Event<'a, T> {
 /// An event that represents some aspect of a room's state.
 pub trait StateEvent<'a, 'b, T>: RoomEvent<'a, T> {
     /// Previous content for this aspect of room state.
-    fn prev_content(&'a self) -> Option<&'b T>;
+    fn prev_content(&'a self) -> Option<&'b T> {
+        None
+    }
+
     /// A unique key which defines the overwriting semantics for this aspect of room state.
-    fn state_key(&self) -> &'a str;
+    fn state_key(&self) -> &'a str {
+        ""
+    }
 }