Makes combat tables for items refer back to their source item.

Adds a self property to o.combat and o.special_combat that refers back to item's full table. Combat tables tend to get passed around through various functions and callbacks and it can be difficult to connect a combat table to a full object. This makes the link two-way (o.combat.self == o).

This doesn't break saves, but any functionality dependent on the link will require a new save, as it only applies to newly generated items.