Add utilty methods/functions for getting Directions

Assets/DanmakU/Runtime/Core/Danmaku.cs

@@ -65,6 +65,11 @@ public float Rotation {
     [MethodImpl(MethodImplOptions.AggressiveInlining)] set { Pool.Rotations[Id] = value; }
   }
 
+  /// <summary>
+  /// Gets the direction the Danmaku is facing. Guarenteed to be a unit vector.
+  /// </summary>
+  public Vector2 Direction => GetDirection(Pool.Rotations[Id]);
+
   /// <summary>
   /// Gets or sets the speed of the Danmaku.
   /// </summary>
@@ -154,6 +159,15 @@ public void ApplyState(DanmakuState state) {
     Color = state.Color;
   }
 
+  /// <summary>
+  /// Convert a Danmaku rotation into a unit vector.
+  /// </summary>
+  /// <param name="rotation">the rotation of the Danmaku.</param>
+  /// <returns>the unit vector representing the way the bullet is facing.</returns>
+  public static Vector2 GetDirection(float rotation) {
+    return new Vector2(Mathf.Cos(rotation), Mathf.Sin(rotation));
+  }
+
   public static bool operator ==(Danmaku lhs, Danmaku rhs) {
     return lhs.Id == rhs.Id && lhs.Pool == rhs.Pool;
   }