Change all of body system to use entities and components (#2074)

* Early commit

* Early commit 2

* merging master broke my git

* does anyone even read these

* life is fleeting

* it just works

* this time passing integration tests

* Remove hashset yaml serialization for now

* You got a license for those nullables?

* No examine, no context menu, part and mechanism parenting and visibility

* Fix wrong brain sprite state

* Removing layers was a mistake

* just tear body system a new one and see if it still breathes

* Remove redundant code

* Add that comment back

* Separate damage and body, component states, stomach rework

* Add containers for body parts

* Bring layers back pls

* Fix parts magically changing color

* Reimplement sprite layer visibility

* Fix tests

* Add leg test

* Active legs is gone

Crab rave

* Merge fixes, rename DamageState to CurrentState

* Remove IShowContextMenu and ICanExamine

Content.Shared/GameObjects/Components/Body/BodyExtensions.cs

@@ -0,0 +1,29 @@
+#nullable enable
+using System.Diagnostics.CodeAnalysis;
+using Robust.Shared.Interfaces.GameObjects;
+
+namespace Content.Shared.GameObjects.Components.Body
+{
+    public static class BodyExtensions
+    {
+        public static T? GetBody<T>(this IEntity entity) where T : class, IBody
+        {
+            return entity.GetComponentOrNull<T>();
+        }
+
+        public static bool TryGetBody<T>(this IEntity entity, [NotNullWhen(true)] out T? body) where T : class, IBody
+        {
+            return (body = entity.GetBody<T>()) != null;
+        }
+
+        public static IBody? GetBody(this IEntity entity)
+        {
+            return entity.GetComponentOrNull<IBody>();
+        }
+
+        public static bool TryGetBody(this IEntity entity, [NotNullWhen(true)] out IBody? body)
+        {
+            return (body = entity.GetBody()) != null;
+        }
+    }
+}