Fixes for Arceus typing

Fix Arceus overriding Trick-or-Treat and Forest´s Curse.
Implement Arceus typing more reliably by setting it when it switches into the field.
Display fail messages when trying to modify Arceus´ first type by any means.
Also fix mons keeping their typing when changing to another forme.
pokemon.setType doesn't allow multiple types as parameters anymore, but if its second parameter is true, then it will ignore whether the mon is Arceus or not.