Fix UnityDebugLogger implementation (#143)

## Description of Changes

Without explicit reference these result in

> error CS0119: 'UnityDebugLogger.Debug(string)' is a method, which is
not valid in the given context

## API

 - [ ] This is an API breaking change to the SDK

*If the API is breaking, please state below what will break*


## Requires SpacetimeDB PRs
*List any PRs here that are required for this SDK change to work*

src/UnityDebugLogger.cs

@@ -4,46 +4,31 @@
  */
 #if UNITY_5_3_OR_NEWER
 using System;
-using UnityEngine;
 
 namespace SpacetimeDB
 {
     internal class UnityDebugLogger : ISpacetimeDBLogger
     {
-        public void Debug(string message)
-        {
-            Debug.Log(message);
-        }
+        public void Debug(string message) =>
+            UnityEngine.Debug.Log(message);
 
-        public void Trace(string message)
-        {
-            Debug.Log(message);
-        }
+        public void Trace(string message) =>
+            UnityEngine.Debug.Log(message);
 
-        public void Info(string message)
-        {
-            Debug.Log(message);
-        }
+        public void Info(string message) =>
+            UnityEngine.Debug.Log(message);
 
-        public void Warn(string message)
-        {
-            Debug.LogWarning(message);
-        }
+        public void Warn(string message) =>
+            UnityEngine.Debug.LogWarning(message);
 
-        public void Error(string message)
-        {
-            Debug.LogError(message);
-        }
+        public void Error(string message) =>
+            UnityEngine.Debug.LogError(message);
 
-        public void Exception(string message)
-        {
-            Debug.LogError(message);
-        }
+        public void Exception(string message) =>
+            UnityEngine.Debug.LogError(message);
 
-        public void Exception(Exception e)
-        {
-            Debug.LogException(e);
-        }
+        public void Exception(Exception e) =>
+            UnityEngine.Debug.LogException(e);
     }
 }
 #endif