Added IForgeCustomInventory.listContents and InventoryAdapters.asCustomInventory(IForgeLinearInventory).
Other small changes and bugfixes, notably:
* It is not mandatory for mods to obey ILinearInventorySlot.shouldInsertItem/shouldExtractItems.
* VanillaSidedSlotAdapter.setStack doesn't check ISidedInventory.func_102008_b - the vanilla method that checks if an item can be extracted from a slot. shouldExtractItems still checks this.