Add utilities for fetching lists of entities

lib/nyxx_extensions.dart

@@ -30,3 +30,4 @@ export 'src/extensions/role.dart';
 export 'src/extensions/scheduled_event.dart';
 export 'src/extensions/snowflake_entity.dart';
 export 'src/extensions/user.dart';
+export 'src/extensions/list.dart';

lib/src/extensions/list.dart

@@ -0,0 +1,11 @@
+import 'package:nyxx/nyxx.dart';
+
+/// Extensions for fetching lists of [SnowflakeEntity]s.
+extension PartialList<T extends SnowflakeEntity<T>>
+    on List<SnowflakeEntity<T>> {
+  /// Get all the entities in this list using the cached entity if possible.
+  Future<List<T>> get() => Future.wait(map((entity) => entity.get()));
+
+  /// Fetch all the entities in this list.
+  Future<List<T>> fetch() => Future.wait(map((entity) => entity.fetch()));
+}