provide a way to enumerate entities managed by the DataStack

CoreStore/Setting Up/DataStack.swift

@@ -81,6 +81,14 @@ public final class DataStack {
         return self.model.currentModelVersion!
     }
     
+    /**
+    Returns the entity name-to-class type mapping from the `DataStack`'s model.
+    */
+    public var entitiesByName: [String: NSManagedObject.Type] {
+        
+        return self.model.entityMapping()
+    }
+    
     /**
     Adds an in-memory store to the stack.