unsafeify World::entities_mut (#4093)

# Objective
make bevy ecs a lil bit less unsound

## Solution
make unsound API unsafe so that there is an unsafe block to blame:

```rust
use bevy_ecs::prelude::*;

#[derive(Debug, Component)]
struct Foo(u8);

fn main() {
    let mut world = World::new();
    let e1 = world.spawn().id();
    let e2 = world.spawn().insert(Foo(2)).id();
    world.entities_mut().meta[0] = world.entities_mut().meta[1].clone();
    let foo = world.entity(e1).get::<Foo>().unwrap();
    // whoo i love having components i dont have
    dbg!(foo);
}
```

This is not _strictly_ speaking UB, however: 
- `Query::get_multiple` cannot work if this is allowed
- bevy_ecs is a pile of unsafe code whose soundness generally depends on the world being in a "correct" state with "no funny business" so it seems best to disallow this
- it is trivial to get bevy to panic inside of functions with safety invariants that have been violated (the entity location is not valid)
- it seems to violate what the safety invariant on `Entities::flush` is trying to ensure

crates/bevy_ecs/src/entity/mod.rs

@@ -199,7 +199,7 @@ impl<'a> core::iter::ExactSizeIterator for ReserveEntitiesIterator<'a> {}
 
 #[derive(Debug, Default)]
 pub struct Entities {
-    pub meta: Vec<EntityMeta>,
+    pub(crate) meta: Vec<EntityMeta>,
 
     /// The `pending` and `free_cursor` fields describe three sets of Entity IDs
     /// that have been freed or are in the process of being allocated:
@@ -544,6 +544,12 @@ impl Entities {
         }
     }
 
+    /// Accessor for getting the length of the vec in `self.meta`
+    #[inline]
+    pub fn meta_len(&self) -> usize {
+        self.meta.len()
+    }
+
     #[inline]
     pub fn len(&self) -> u32 {
         self.len

crates/bevy_ecs/src/world/mod.rs

@@ -135,8 +135,12 @@ impl World {
     }
 
     /// Retrieves this world's [Entities] collection mutably
+    ///
+    /// # Safety
+    /// Mutable reference must not be used to put the [`Entities`] data
+    /// in an invalid state for this [`World`]
     #[inline]
-    pub fn entities_mut(&mut self) -> &mut Entities {
+    pub unsafe fn entities_mut(&mut self) -> &mut Entities {
         &mut self.entities
     }
 

crates/bevy_render/src/lib.rs

@@ -189,7 +189,7 @@ impl Plugin for RenderPlugin {
 
                     // reserve all existing app entities for use in render_app
                     // they can only be spawned using `get_or_spawn()`
-                    let meta_len = app_world.entities().meta.len();
+                    let meta_len = app_world.entities().meta_len();
                     render_app
                         .world
                         .entities()
@@ -198,7 +198,7 @@ impl Plugin for RenderPlugin {
                     // flushing as "invalid" ensures that app world entities aren't added as "empty archetype" entities by default
                     // these entities cannot be accessed without spawning directly onto them
                     // this _only_ works as expected because clear_entities() is called at the end of every frame.
-                    render_app.world.entities_mut().flush_as_invalid();
+                    unsafe { render_app.world.entities_mut() }.flush_as_invalid();
                 }
 
                 {