In single-player multiworlds with small item pools, Noita was manually
placing some items into Shop Item locations, but was only setting
location.item, and not also setting item.location so that the item and
location refer to one another.
This has been fixed by using the MultiWorld.push_item() helper method to
place the items instead of manually placing the items.
Reworking the options to make it work with the new options API.
Also reworked stuff in several spots to use world: NoitaWorld instead of multiworld: MultiWorld
