Working with CUtlVector: loadout slot edition

Added

• Native TF2Econ_TranslateLoadoutSlotIndexToName, which takes in a loadout slot index and a buffer to store the slot name (corresponds to the item_slot key on items).
• Native TF2Econ_TranslateLoadoutSlotNameToIndex, which performs the inverse operation; given a slot name it returns the slot index.
• Native TF2Econ_GetLoadoutSlotCount, which returns the number of current valid loadout slots.