Miscellaneous Unity Fixes (#3)

* Use simplified attribute names and ensure function calls are made with CallingConvention.Cdecl

* Remove unused imports

examples/button-clicker/Assets/DiscordController.cs

@@ -1,6 +1,4 @@
-using System.Collections;
-using System.Collections.Generic;
-using UnityEngine;
+using UnityEngine;
 
 public class DiscordController : MonoBehaviour {
     public DiscordRpc.RichPresence presence;