Add utilities for fetching lists of entities (#42)

lib/nyxx_extensions.dart

@@ -30,4 +30,5 @@ 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';
 export 'src/extensions/application.dart';

lib/src/extensions/list.dart

@@ -0,0 +1,10 @@
+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()));
+}

test/unit/sanitizer_test.dart

@@ -5,7 +5,8 @@ import 'package:test/test.dart';
 
 const _whitespaceCharacter = "‎";
 
-final sampleContent = '<@1234> test <@!2345> test2 <@&3456> test3 <#4567> test4 <a:test_emoji:5678> test5 <:test_emoji:6789> test6 </test command:123456789123456789>';
+final sampleContent =
+    '<@1234> test <@!2345> test2 <@&3456> test3 <#4567> test4 <a:test_emoji:5678> test5 <:test_emoji:6789> test6 </test command:123456789123456789>';
 final removed = ' test  test2  test3  test4  test5  test6 ';
 final sanitized =
     '<@${_whitespaceCharacter}1234> test <@${_whitespaceCharacter}2345> test2 <@&${_whitespaceCharacter}3456> test3 <#${_whitespaceCharacter}4567> test4 <${_whitespaceCharacter}a\\:test_emoji\\:5678> test5 <$_whitespaceCharacter\\:test_emoji\\:6789> test6 </${_whitespaceCharacter}test command:123456789123456789>';