this eliminates the performance issues described by #5719.
closes#5719
we may want to consider exposing a public API for this in the future, since it might be useful for plugins.
avoid the overhead incurred by clear() and setItem(), because in internalSetContents(), we already have no listeners or viewers to talk to anyway, so this is just spamming shit into /dev/null.
we don't usually add VanillaItems entries for blocks since they already exist in VanillaBlocks, but air has a special use case specifically as an itemstack, so we make an exception for this case.
previously, these were forced to extend BaseInventory because of the amount of crap in Inventory's interface.
This meant that these inventories had their own slots storage, which would be _mostly_ unused because these inventories aren't real inventories, but rather just delegates.
This lead to a variety of bugs in the past, such as certain API methods on BaseInventory not working correctly for DoubleChestInventory in particular.
Now, BaseInventory just implements the functional part of the inventory implementation, leaving the storage system up to the implementation.
A SimpleInventory class is provided with a simple SplFixedArray storage backing, which is used by most inventories.
EnderChestInventory and DoubleChestInventory now extend BaseInventory directly, and implement custom methods for dealing with their delegates.
