Make objects know what inventory they're in.

Inspired by the mess that is Wintertide's special on hit. As of !693 (merged), combat tables and their objects is now a two-way association. This MR aims to make the object-actor association similarly two way. Items should know where they are and who is holding them, which is accomplished by setting o.in_inven, which holds o.in_inven.actor and o.in_inven.id. This prevents requiring obnoxious searches of inventories for an item in question, and prevents potential search collisions (as define_as is not a perfect identifier).

This MR is save safe, although any behavior reliant on this will not apply to objects without changing their position in inventories.