This refactors the object_h struct to store objects as indexes rather than pointers. This is a prerequisite for making the collection of objects dynamic in the future. See also #6100.
Follow-up to #5800 and #5829.
When returning objects, be sure to use `ade_set_object_with_breed`, per the documentation on that function. This ensures that all relevant subclass information can be used.
Also fix the return value of streaminganim's isValid() function.
This maintains the type information which can be used for enforce
const-correctness and also for implementing more advanced ways of
handling the type information.
Goober5000