Add the ability the define inherited hooks.

Creating inheritance hooks that play well with others (e.g. don't clobber inheritance that are already defined) requires injecting inheritance hooks into the Class class (via extend). This works pretty well, but requires a lot of setup when typically, only a few lines need to be executed (i.e. eval'd) on the subclass.

To simplify this add, an `on_inherited` method that captures code and executes when the class is inherited.