Summary:
## Changes in this pull request
### Checklist
- [x] All tests pass. Demo project builds and runs.
- [x] I added tests, an experiment, or detailed why my change isn't tested.
- [x] I added an entry to the `CHANGELOG.md` for any breaking changes, enhancements, or bug fixes.
- [x] I have reviewed the [contributing guide](https://github.com/Instagram/IGListKit/blob/master/.github/CONTRIBUTING.md)
Pull Request resolved: https://github.com/Instagram/IGListKit/pull/1466
Reviewed By: bdotdub
Differential Revision: D24235336
Pulled By: lorixx
fbshipit-source-id: bfc5fab26ae28ff9306d1b889b82b49c6ae2a636
Summary:
This diff introduces two experimental APIs to make `IGListKit` more usable with Swift values types:
- The `ListIdentifiable` protocol, which provides the equivalent of the diff identifier half of `ListDiffable`. For equality, we just use Swift's native `Equatable`.
- The `ListValueSectionController` base class, which provides a section controller interface for use with `ListIdentifiable` values.
These two APIs work together through the use of a private box class. I'd like to keep this detail hidden from callers, so that product code only deals with native Swift types.
Differential Revision: D23606413
fbshipit-source-id: 23718c508643f165c74550f1515d77448bd42d42
