Layout invalidation API

Summary:
Adding a new layout-invalidation API, telling the layout object to query and rebuild the layout for all items in the section controller. This works with `UICollectionViewFlowLayout` and should work with other custom layouts (including our own).

Issue fixed: #360, #459

- [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.
Closes https://github.com/Instagram/IGListKit/pull/499

Reviewed By: jessesquires

Differential Revision: D4590274

Pulled By: rnystrom

fbshipit-source-id: f87235be4e6c024bf979b831a8938be68895e011

Examples/Examples-iOS/IGListKitExamples/SectionControllers/ExpandableSectionController.swift

@@ -32,7 +32,6 @@ final class ExpandableSectionController: IGListSectionController, IGListSectionT
 
     func cellForItem(at index: Int) -> UICollectionViewCell {
         let cell = collectionContext!.dequeueReusableCell(of: LabelCell.self, for: self, at: index) as! LabelCell
-        cell.label.numberOfLines = expanded ? 0 : 1
         cell.label.text = object
         return cell
     }
@@ -43,7 +42,14 @@ final class ExpandableSectionController: IGListSectionController, IGListSectionT
 
     func didSelectItem(at index: Int) {
         expanded = !expanded
-        collectionContext?.reload(in: self, at: IndexSet(integer: 0))
+        UIView.animate(withDuration: 0.5,
+                       delay: 0,
+                       usingSpringWithDamping: 0.4,
+                       initialSpringVelocity: 0.6,
+                       options: [],
+                       animations: {
+                        self.collectionContext?.invalidateLayout(for: self)
+        })
     }
 
 }