Potential fix for invalidation crashes

As mentioned in #13 and #8 there are some scenarios where the collection view’s data is out-of-sync with the data in composed.

As mentioned in #13 calling `layoutIfNeeded` can trigger the data to be in sync again. In this I have added it to `mappingWillBeginUpdating(_:)` which _appears_ to solve the problem.

It might be needed in `replace(sectionProvider:)` (because `reloadData` is called) and/or `mappingDidInvalidate(_:)` (for the same reason) but I’m still investigating.

Author: JosephDuffy

Sources/ComposedUI/CollectionView/CollectionCoordinator.swift

@@ -217,6 +217,7 @@ extension CollectionCoordinator: SectionProviderMappingDelegate {
     public func mappingWillBeginUpdating(_ mapping: SectionProviderMapping) {
         reset()
         defersUpdate = true
+        collectionView.layoutIfNeeded()
     }
 
     public func mappingDidEndUpdating(_ mapping: SectionProviderMapping) {