Mark TableRow and TableId as repr(transparent) (#7166)

# Objective
Following #6681, both `TableRow` and `TableId` are now part of `EntityLocation`. However, the safety invariant on `EntityLocation` requires that all of the constituent fields are `repr(transprent)` or `repr(C)` and the bit pattern of all 1s must be valid. This is not true for `TableRow` and `TableId` currently.

## Solution
Mark `TableRow` and `TableId` to satisfy the safety requirement. Add safety comments on `ArchetypeId`, `ArchetypeRow`, `TableId` and `TableRow`.

crates/bevy_ecs/src/archetype.rs

@@ -40,6 +40,7 @@ use std::{
 /// [`World`]: crate::world::World
 /// [`Entities::get`]: crate::entity::Entities
 #[derive(Debug, Copy, Clone, Eq, PartialEq)]
+// SAFETY: Must be repr(transparent) due to the safety requirements on EntityLocation
 #[repr(transparent)]
 pub struct ArchetypeRow(u32);
 
@@ -68,6 +69,7 @@ impl ArchetypeRow {
 /// [`World`]: crate::world::World
 /// [`EMPTY`]: crate::archetype::ArchetypeId::EMPTY
 #[derive(Debug, Copy, Clone, Eq, PartialEq, Hash)]
+// SAFETY: Must be repr(transparent) due to the safety requirements on EntityLocation
 #[repr(transparent)]
 pub struct ArchetypeId(u32);
 

crates/bevy_ecs/src/storage/table.rs

@@ -26,6 +26,8 @@ use std::{
 /// [`Archetype`]: crate::archetype::Archetype
 /// [`Archetype::table_id`]: crate::archetype::Archetype::table_id
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
+// SAFETY: Must be repr(transparent) due to the safety requirements on EntityLocation
+#[repr(transparent)]
 pub struct TableId(u32);
 
 impl TableId {
@@ -64,6 +66,8 @@ impl TableId {
 /// [`Archetype::table_id`]: crate::archetype::Archetype::table_id
 /// [`Entity`]: crate::entity::Entity
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
+// SAFETY: Must be repr(transparent) due to the safety requirements on EntityLocation
+#[repr(transparent)]
 pub struct TableRow(u32);
 
 impl TableRow {