Add RemovedComponentEvents::iter (#12815)

# Objective

Sometimes it's useful to iterate over removed entities. For example, in
my library
[bevy_replicon](https://github.com/projectharmonia/bevy_replicon) I need
it to iterate over all removals to replicate them over the network.

Right now we do lookups, but it would be more convenient and faster to
just iterate over all removals.

## Solution

Add `RemovedComponentEvents::iter`.

---

## Changelog

### Added

- `RemovedComponentEvents::iter` to iterate over all removed components.

---------

Co-authored-by: Pablo Reinhardt <126117294+pablo-lua@users.noreply.github.com>

crates/bevy_ecs/src/removal_detection.rs

@@ -83,6 +83,11 @@ impl RemovedComponentEvents {
         }
     }
 
+    /// Returns an iterator over components and their entity events.
+    pub fn iter(&self) -> impl Iterator<Item = (&ComponentId, &Events<RemovedComponentEntity>)> {
+        self.event_sets.iter()
+    }
+
     /// Gets the event storage for a given component.
     pub fn get(
         &self,