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.

See merge request !693