Put the class name on the stack instead of as an arg.

src/wren_vm.h

@@ -168,9 +168,9 @@ typedef enum
   // Pushes the created closure.
   CODE_CLOSURE,
 
-  // TODO: Doc.
-  // Define a new empty class and push it. Short [arg1] is a constant for the
-  // name of the class. Byte [arg2] is the number of fields in the class.
+  // Creates a class. Top of stack is the superclass, or `null` if the class
+  // inherits Object. Below that is a string for the name of the class. Byte
+  // [arg] is the number of fields in the class.
   CODE_CLASS,
 
   // Define a method for symbol [arg]. The class receiving the method is popped