Calls func for all instantiable subtypes of parent_type.
This is often useful for extending functionality by way of Module. A module may register a
subtype of parent_type in its e_module_load function. Then later on the application will call
type_traverse to instantiate all registered subtypes of parent_type.
| parent_type |
the root Type to traverse from |
| func |
the function to call for each visited Type |
| user_data |
user data to pass to the function |