Process #initialize of all classes first

[yhara]

• CI fail on Ubuntu https://github.com/yhara/shiika/pull/232/checks?check_run_id=1676197864
• On mac, it can be reproduced by renaming builtin/_tuple.sk to builtin/zz.sk

ProgramError: method MethodFirstname("fst") not found on ClassFullname("Pair")

To fix this, #initialize of all classes should be processed before processing other methods (because attr getters/setters are defined by processing #initialize.)