Handle Unary operator overloading for UserData

UpsilonTests/UserDataOperatorTests.cs

@@ -46,6 +46,11 @@ namespace UpsilonTests
                 return new UserDataHelper(a.Value - b.Value);
             }
 
+            public static UserDataHelper operator -(UserDataHelper a)
+            {
+                return new UserDataHelper(-a.Value);
+            }
+
             public static UserDataHelper operator *(UserDataHelper a, UserDataHelper b)
             {
                 return new UserDataHelper(a.Value * b.Value);
@@ -145,5 +150,21 @@ end
             Assert.Equal(10, result.Value);
         }
 
+        [Fact]
+        public void TestNegation()
+        {
+            const string input = @"
+function negate(o1)
+    return -o1
+end
+";
+            var script = new Script(input);
+            Assert.Empty(script.Diagnostics.Messages);
+            var o1     = new UserDataHelper(100);
+            var result = script.EvaluateFunction<UserDataHelper>("negate", new object[] {o1});
+            Assert.Empty(script.Diagnostics.Messages);
+            Assert.Equal(-100, result.Value);
+        }
+
     }
 }