Move getPlayerItemById to compat.lua
Fix bug in getPlayerSlotItem
Move getContainerItem to compat.lua
Move doAddContainerItemEx to compat.lua
Remove unnecessary env->addThing calls

There is no point in adding the item to the local UID map if we're not
exposing it to the user anyway because the user won't know the UID to
utilize it. To know the UID, you must call item:getUniqueId() which
already calls env->addThing.