Problem: Getting an item from a NULL dict crashes. Setting a register to a
NULL list crashes. (Nikolai Pavlov, issue #768) Comparing a NULL
dict with a NULL dict fails.
Solution: Properly check for NULL.
Problem: Functions specifically for testing do not sort together.
Solution: Rename garbagecollect_for_testing() to test_garbagecollect_now().
Add test_null_list(), test_null_dict(), etc.
Problem: Comparing a function that exists on two dicts is not backwards
compatible. (Thinca)
Solution: Only compare the function, not what the partial adds.
