exact sized event iterators (#3863)

# Objective

- Remove `Resource` binding on events, introduce a new `Event` trait
- Ensure event iterators are `ExactSizeIterator`

## Solution

- Builds on #2382 and #2969

## Changelog

- Events<T>, EventWriter<T>, EventReader<T> and so on now require that the underlying type is Event, rather than Resource. Both of these are trivial supertraits of Send + Sync + 'static with universal blanket implementations: this change is currently purely cosmetic.
- Event reader iterators now implement ExactSizeIterator

Author: mockersf

crates/bevy_app/src/app.rs

@@ -1,7 +1,7 @@
 use crate::{CoreStage, Plugin, PluginGroup, PluginGroupBuilder, StartupSchedule, StartupStage};
 pub use bevy_derive::AppLabel;
 use bevy_ecs::{
-    event::Events,
+    event::{Event, Events},
     prelude::{FromWorld, IntoExclusiveSystem},
     schedule::{
         IntoSystemDescriptor, Schedule, ShouldRun, Stage, StageLabel, State, StateData, SystemSet,
@@ -622,7 +622,7 @@ impl App {
     /// ```
     pub fn add_event<T>(&mut self) -> &mut Self
     where
-        T: Resource,
+        T: Event,
     {
         if !self.world.contains_resource::<Events<T>>() {
             self.init_resource::<Events<T>>()