Merge branch 'more_self_referential_inventory' into 'master'

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.**

See merge request !802