Make objects know what inventory they're in.
Inspired by the mess that is Wintertide's special on hit. As of !693, 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.
Merged
by 
The changes were merged into master. The source branch has been removed.
