This stores the components as children of the item that contains them via the DB's `parent_container_item_id` feature, and ensures that things are loaded in a good order with breadth-first search.
Squahed fixes:
- Fix some constraint violations that occurred when swapping inventory items.
- Comment out recipes for modular weapons.
- Make update_item_at_slot_using_persistence_key and is_modular more idiomatic.
- Add changelog entry.
- Document `defer_foreign_keys` usage.
- Add ItemKind::ModularComponent, Item::components.
- Add tool::StatKind::{Direct,Modular} for the modular weapons themselves.
- Move ItemConfig from ItemDesc to Item, so components' stats can be taken into account.
- Crafting stores into the components field.
- Components/recipes/placeholders are created dynamically.
- Show which components a modular weapon contains in the tooltip in voxygen.
Squashed fixes:
- `Item::duplicate` components in `Item::new_from_item_def`.
- Speed of 1.0 for now.
