Summary

SUMMARY: Infrastructure "encapsulate item::contents into a new class"

Purpose of change

encapsulation will help enormously with continuing to extend item::contents for the nested container system. it'll also restrict accessing contents directly all over the place, like it is now.

Describe the solution

creates a new class called item_contents that has the item contents. adds functions to interact with them. You might notice that a few functions have ret_val returns with not really much going on there, but that is because i'm going to be expanding them basically immediately after, so i saw no reason to redo the function call sites twice. i also had to remove item::emplace_back() and alter a bunch of places that should have item::put_in or something similar. additionally, i have item_contents::front() and item_contents::back() to aid further work, with intent to remove these functions.

Relies on #38022 and has the commits from that pr

Testing

Compiled successfully. additional testing TBD; i altered some tests so they'll show up on travis, and i'll run the game to make sure save/load works properly.