Let Bukkit serialize (some) inventories...

Since ConfigurationSerializable, there's no need to manually serialize
items anymore, and thus we let Bukkit serialize player inventories for
when players join arenas.

However, the "repairables" in the soft-restore mechanism still use the
now obsolete SerializableItem and SerializableInventory classes, and
should perhaps be rewritten.

Note that for 1.0, it may still be necessary to write a custom means
of serializing items, since we don't know if a given implementation
supports native serialization (however it should be possible to rely
on Bukkit's in the Bukkit-specific classes).