Represent Inventories as a slot-entity map
Previously inventories stored their contents as a list of entities.
This proved to be a mistake (see ocelot-desktop#27 (closed)).
This MR reimplements the Inventory
trait as a bidirectional map of slot indices to entities.
The NBT save format has been affected by the changes. Both the old, slotless, format and the new format can be loaded; when saving, however, only the new format is available.