Clean internal NSArray copying

Summary:
Fixed #384

- [x] All tests pass. Demo project builds and runs.
Closes https://github.com/Instagram/IGListKit/pull/441

Differential Revision: D4448500

Pulled By: rnystrom

fbshipit-source-id: 38276083a3689eea92c1c7afd1bded3c8fe96282

Source/IGListAdapter.m

@@ -373,7 +373,7 @@
 - (NSArray *)objects {
     IGAssertMainThread();
 
-    return [self.sectionMap.objects copy];
+    return self.sectionMap.objects;
 }
 
 - (id<IGListSupplementaryViewSource>)supplementaryViewSourceAtIndexPath:(NSIndexPath *)indexPath {
@@ -492,7 +492,7 @@
     // clear the view controller and collection context
     IGListSectionControllerPopThread();
 
-    [map updateWithObjects:objects sectionControllers:[sectionControllers copy]];
+    [map updateWithObjects:objects sectionControllers:sectionControllers];
 
     // now that the maps have been created and contexts are assigned, we consider the section controller "fully loaded"
     for (id object in updatedObjects) {
@@ -555,7 +555,7 @@
             [indexPaths addObject:[NSIndexPath indexPathForItem:idx inSection:section]];
         }];
     }
-    return [indexPaths copy];
+    return indexPaths;
 }
 
 - (NSIndexPath *)indexPathForSectionController:(IGListSectionController *)controller index:(NSInteger)index {
@@ -584,7 +584,7 @@
         }
     }
 
-    return [attributes copy];
+    return attributes;
 }
 
 - (void)mapCell:(UICollectionViewCell *)cell toSectionController:(IGListSectionController<IGListSectionType> *)sectionController {
@@ -784,7 +784,7 @@
             [cells addObject:cell];
         }
     }
-    return [cells copy];
+    return cells;
 }
 
 - (void)deselectItemAtIndex:(NSInteger)index

Source/IGListAdapterUpdater.m

@@ -105,7 +105,7 @@ static NSArray *objectsWithDuplicateIdentifiersRemoved(NSArray<id<IGListDiffable
             IGLKLog(@"WARNING: Object %@ already appeared in objects array", object);
         }
     }
-    return [uniqueObjects copy];
+    return uniqueObjects;
 }
 
 - (void)performBatchUpdatesWithCollectionView:(UICollectionView *)collectionView {
@@ -274,8 +274,8 @@ void convertReloadToDeleteInsert(NSMutableIndexSet *reloads,
     // reloadSections: is unsafe to use within performBatchUpdates:, so instead convert all reloads into deletes+inserts
     convertReloadToDeleteInsert(reloads, deletes, inserts, diffResult, fromObjects);
 
-    IGListBatchUpdateData *updateData = [[IGListBatchUpdateData alloc] initWithInsertSections:[inserts copy]
-                                                                               deleteSections:[deletes copy]
+    IGListBatchUpdateData *updateData = [[IGListBatchUpdateData alloc] initWithInsertSections:inserts
+                                                                               deleteSections:deletes
                                                                                  moveSections:moves
                                                                              insertIndexPaths:insertIndexPaths
                                                                              deleteIndexPaths:deleteIndexPaths

Source/IGListGridCollectionViewLayout.m

@@ -314,7 +314,7 @@
         x += size.width + self.minimumInteritemSpacing;
         idx++;
     }
-    return [array copy];
+    return array;
 }
 
 #pragma mark - Private API

Source/IGListStackedSectionController.m

@@ -184,7 +184,7 @@ static void * kStackedSectionControllerIndexKey = &kStackedSectionControllerInde
             [cells addObject:cell];
         }
     }
-    return [cells copy];
+    return cells;
 }
 
 - (void)deselectItemAtIndex:(NSInteger)index sectionController:(IGListSectionController<IGListSectionType> *)sectionController animated:(BOOL)animated {