[ListMonitor] [numberOfObjects()] Return count of fetchedObjects.

Casting `fetchedObjects` to `NSArray?` has better performance.
https://github.com/JohnEstropia/CoreStore/pull/288

Sources/ListMonitor.swift

@@ -232,10 +232,7 @@ public final class ListMonitor<D: DynamicObject>: Hashable {
             !self.isPendingRefetch || Thread.isMainThread,
             "Attempted to access a \(cs_typeName(self)) outside the main thread while a refetch is in progress."
         )
-        return self.sections().reduce(0, { (numberOfObjects, sectionInfo) -> Int in
-            
-            return numberOfObjects + sectionInfo.numberOfObjects
-        })
+        return (self.fetchedResultsController.fetchedObjects as NSArray?)?.count ?? 0
     }
     
     /**