Make entity generation a new type and remove identifier (#19121)

# Objective

This is a followup to #18704 . There's lots more followup work, but this
is the minimum to unblock #18670, etc.

This direction has been given the green light by Alice
[here](#18704 (comment)).

## Solution

I could have split this over multiple PRs, but I figured skipping
straight here would be easiest for everyone and would unblock things the
quickest.

This removes the now no longer needed `identifier` module and makes
`Entity::generation` go from `NonZeroU32` to `struct
EntityGeneration(u32)`.

## Testing

CI

---------

Co-authored-by: Mark Nokalt <marknokalt@live.com>

Author: ElliottjPierce

benches/benches/bevy_ecs/world/entity_hash.rs

@@ -1,4 +1,4 @@
-use bevy_ecs::entity::{Entity, EntityHashSet};
+use bevy_ecs::entity::{Entity, EntityGeneration, EntityHashSet};
 use criterion::{BenchmarkId, Criterion, Throughput};
 use rand::{Rng, SeedableRng};
 use rand_chacha::ChaCha8Rng;
@@ -21,7 +21,10 @@ fn make_entity(rng: &mut impl Rng, size: usize) -> Entity {
     let bits = ((generation as u64) << 32) | id;
     let e = Entity::from_bits(bits);
     assert_eq!(e.index(), !(id as u32));
-    assert_eq!(e.generation(), generation as u32);
+    assert_eq!(
+        e.generation(),
+        EntityGeneration::FIRST.after_versions(generation as u32)
+    );
     e
 }
 

crates/bevy_ecs/src/entity/map_entities.rs

@@ -3,7 +3,6 @@ use indexmap::IndexSet;
 
 use crate::{
     entity::{hash_map::EntityHashMap, Entity},
-    identifier::masks::{IdentifierMask, HIGH_MASK},
     world::World,
 };
 
@@ -229,11 +228,9 @@ impl EntityMapper for SceneEntityMapper<'_> {
         // this new entity reference is specifically designed to never represent any living entity
         let new = Entity::from_raw_and_generation(
             self.dead_start.row(),
-            IdentifierMask::inc_masked_high_by(self.dead_start.generation, self.generations),
+            self.dead_start.generation.after_versions(self.generations),
         );
-
-        // Prevent generations counter from being a greater value than HIGH_MASK.
-        self.generations = (self.generations + 1) & HIGH_MASK;
+        self.generations = self.generations.wrapping_add(1);
 
         self.map.insert(source, new);