An inventory that doesn't fight you

Add an inventory to my game. Items have <describe — name, type,
stackable?>.

- The data structure, and why that one.
- Adding an item when the inventory is full, or partly full for a
  stack — split it, or refuse it? Say which and be consistent.
- Removing a specific quantity across multiple stacks.
- Sorting and filtering.
- Serialising for a save, in a way that survives me adding new item
  types later.

Handle these explicitly: adding zero, adding more than the stack
limit, removing more than I have, and an item type that no longer
exists in a loaded save.