Added core.h config ENABLE_ITEM_GUID
* Enabled by default to take effect of `guid` function.
* When it's disabled, `guid` function will be ignored as suggestion to disable this 'weird. official feature.
* NOTE: If this feature is disabled "in the middle" of your game, the separated is still separated in inventory, storage, or guild storage until player move the item to/from storage/inventory to restack them.

Signed-off-by: Cydh Ramdh <house.bad@gmail.com>