Implemented obtainable enemy item drops

-Added obtainable enemy item drops upon death:
When killed by a player unit, the enemy unit's equipped item will be moved to active player unit's inventory provided the player unit has free slots.

-Added hasSlots function:
returns true if unit has free space in inventory, otherwise returns false

-Added freeSlot function:
returns the first index of unit inventory that is free.

-Hero no longer starts with a full inventory for the sake of this feature.