Adding HelloWorld constraint + sample scene and minor using clean up

Assets/Constraints/ConstraintsUtils.cs

 using System.Collections.Generic;
-using UnityEditor;
-using Unity.Collections;
 
 namespace UnityEngine.Animations.Rigging
 {

Assets/Constraints/TwistChain.cs

 using System.Collections.Generic;
-using UnityEditor;
 using Unity.Collections;
 
 namespace UnityEngine.Animations.Rigging

Assets/Constraints/TwistChainStep0.cs

 using System.Collections.Generic;
-using UnityEditor;
 using Unity.Collections;
 
 namespace UnityEngine.Animations.Rigging

Assets/Constraints/TwistChainStep1.cs

 using System.Collections.Generic;
-using UnityEditor;
 using Unity.Collections;
 
 namespace UnityEngine.Animations.Rigging

Assets/Constraints/HelloWorld.cs

+using UnityEngine;
+using UnityEngine.Animations.Rigging;
+using UnityEngine.Experimental.Animations;
+using Unity.Burst;
+using Unity.Mathematics;
+
+[BurstCompile]
+public struct HelloWorldJob : IWeightedAnimationJob
+{
+    public ReadWriteTransformHandle constrained;
+    public ReadOnlyTransformHandle  source;
+    
+    public FloatProperty jobWeight { get; set; }
+
+    public void ProcessRootMotion(AnimationStream stream) { }
+
+    public void ProcessAnimation(AnimationStream stream)
+    {
+        float w = jobWeight.Get(stream);
+        if (w > 0f)
+        {
+            constrained.SetPosition(
+                stream,
+                math.lerp(constrained.GetPosition(stream), -source.GetPosition(stream), w)
+                );
+        }
+    }
+}
+
+[System.Serializable]
+public struct HelloWorldData : IAnimationJobData
+{
+    public Transform constrainedObject;
+    [SyncSceneToStream] public Transform sourceObject;
+
+    public bool IsValid()
+    {
+        return !(constrainedObject == null || sourceObject == null);
+    }
+
+    public void SetDefaultValues()
+    {
+        constrainedObject = null;
+        sourceObject = null;
+    }
+}
+
+public class HelloWorldBinder : AnimationJobBinder<HelloWorldJob, HelloWorldData>
+{
+    public override HelloWorldJob Create(Animator animator, ref HelloWorldData data, Component component)
+    {
+        return new HelloWorldJob()
+        {
+            constrained = ReadWriteTransformHandle.Bind(animator, data.constrainedObject),
+            source = ReadOnlyTransformHandle.Bind(animator, data.sourceObject)
+        };
+    }
+
+    public override void Destroy(HelloWorldJob job) { }
+}
+
+[DisallowMultipleComponent, AddComponentMenu("SIGGRAPH 2019/Hello World")]
+public class HelloWorld : RigConstraint<
+    HelloWorldJob,
+    HelloWorldData,
+    HelloWorldBinder
+    >
+{
+}

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