Hierarchy commandization (#4197)

## Objective
Implement absolute minimum viable product for the changes proposed in bevyengine/rfcs#53.

## Solution

 - Remove public mutative access to `Parent` (Children is already publicly read-only). This includes public construction methods like `Copy`, `Clone`, and `Default`.
 - Remove `PreviousParent`
 - Remove `parent_update_system`
 - Update all hierarchy related commands to immediately update both `Parent` and `Children` references.

## Remaining TODOs

 - [ ] Update documentation for both `Parent` and `Children`. Discourage using `EntityCommands::remove`
 - [x] Add `HierarchyEvent` to notify listeners of hierarchy updates. This is meant to replace listening on `PreviousParent`

## Followup

 - These changes should be best moved to the hooks mentioned in #3742.
 - Backing storage for both might be best moved to indexes mentioned in the same relations.

crates/bevy_animation/src/lib.rs

@@ -15,7 +15,7 @@ use bevy_ecs::{
     schedule::ParallelSystemDescriptorCoercion,
     system::{Query, Res},
 };
-use bevy_hierarchy::{Children, HierarchySystem};
+use bevy_hierarchy::Children;
 use bevy_math::{Quat, Vec3};
 use bevy_reflect::{Reflect, TypeUuid};
 use bevy_time::Time;
@@ -295,9 +295,7 @@ impl Plugin for AnimationPlugin {
             .register_type::<AnimationPlayer>()
             .add_system_to_stage(
                 CoreStage::PostUpdate,
-                animation_player
-                    .before(TransformSystem::TransformPropagate)
-                    .after(HierarchySystem::ParentUpdate),
+                animation_player.before(TransformSystem::TransformPropagate),
             );
     }
 }