feat(sdk): Find and remove duplicated events in RoomEvents.

This patch uses the new `Deduplicator` type, along with
`LinkeChunk::remove_item_at` to remove duplicated events. When a new
event is received, the older one is removed.

crates/matrix-sdk/src/event_cache/linked_chunk/mod.rs

@@ -948,6 +948,18 @@ impl Position {
     pub fn index(&self) -> usize {
         self.1
     }
+
+    /// Move the index part (see [`Self::index`]) to the left, i.e. subtract 1.
+    ///
+    /// # Panic
+    ///
+    /// This method will panic if it will overflow, i.e. if the index is 0.
+    pub(super) fn move_index_to_the_left(&mut self) {
+        self.1 = self
+            .1
+            .checked_sub(1)
+            .expect("Cannot move position's index to the left because it's already 0");
+    }
 }
 
 /// An iterator over a [`LinkedChunk`] that traverses the chunk in backward