Fix issue where supplementary elements don't track section changes (#404)

* Fix collection view supplementary issue

* Add a fast-path

* Remove the old index path entry unconditionally

* Handle overwriting bug

Source/Private/ASMutableElementMap.h

@@ -21,7 +21,7 @@
 
 NS_ASSUME_NONNULL_BEGIN
 
-@class ASSection, ASCollectionElement;
+@class ASSection, ASCollectionElement, _ASHierarchyChangeSet;
 
 /**
  * This mutable version will be removed in the future. It's only here now to keep the diff small
@@ -47,12 +47,18 @@ AS_SUBCLASSING_RESTRICTED
 
 - (void)removeSectionsOfItems:(NSIndexSet *)itemSections;
 
-- (void)removeSupplementaryElementsInSections:(NSIndexSet *)sections;
-
 - (void)insertEmptySectionsOfItemsAtIndexes:(NSIndexSet *)sections;
 
 - (void)insertElement:(ASCollectionElement *)element atIndexPath:(NSIndexPath *)indexPath;
 
+/**
+ * Update the index paths for all supplementary elements to account for section-level
+ * deletes, moves, inserts. This must be called before adding new supplementary elements.
+ *
+ * This also deletes any supplementary elements in deleted sections.
+ */
+- (void)migrateSupplementaryElementsWithChangeSet:(_ASHierarchyChangeSet *)changeSet;
+
 @end
 
 @interface ASElementMap (MutableCopying) <NSMutableCopying>