Add `sum` method to `List` and `Set`

[ruuda]

Similar to #58, the need for this came up a few times. So far I’ve just been using fold, which works, but is not very ergonomic.

• [x] Ensure documentation is up to date
• [x] Ensure test coverage
• [x] Ensure fuzzer discovers the new code paths (after 1f149b6)
• [x] Ensure changelog is up to date