Generating a runtime error on subclassing of builtins

As discussed in https://github.com/munificent/wren/issues/70, it is not allowed to subclass built-in types.

test/closure/subclassing_closure.wren

@@ -0,0 +1,16 @@
+var f = null
+
+{
+  var a = "a"
+  f = new Fn {
+    IO.print(a)
+  }
+}
+//@FIXME: repair the segfault create a propper test for the subclassing
+
+//var closureSegfault = f
+//class SubClosureSegfault is closureSegfault {} 
+
+//var closureOk = f.call
+//class SubClosureOk is closureOk {} 
+