Adds storeId method on StoredCollection

LeStorage/StoredCollection.swift

@@ -100,6 +100,10 @@ public class StoredCollection<T: Storable>: RandomAccessCollection, SomeCollecti
         return T.resourceName()
     }
     
+    public var storeId: String? {
+        self._store.identifier?.value
+    }
+    
     // MARK: - Loading
     
     /// Migrates if necessary and asynchronously decodes the json file