Add CraftMultiInventory

CraftMultiInventory is meant to be a base class for inventories such as
CraftInventoryAnvil and CraftInventoryCrafting, which are composed of
multiple NMS inventories.

I have not made the changes to CraftInventoryCrafting and
CraftInventoryAnvil as this is meant to simply demonstrate the concept
before actually making the changes.

A potentially better solution would be to simply make these changes to
CraftInventory.  That would make CraftInventory a more suitable base class
for any implementation of Inventory.
Delete CraftMultiInventory and improve CraftInventory.

I decided against trying to clean up CraftInventoryDoubleChest.  The
handling of double chests is simply too messy for me to want to try and
understand how they work at the moment.
Forgot to remove the old version of CraftInventory.setContents
Let CraftInventory.setContents(null) clear an Inventory.

Why not? It's better than a NPE.
Fix bugs.