Refactor the friend namespace (#721)

* Refactor the friend namespace and UInt128

This commit also:
- Fix GetFriendsList arguments ordering.
- Add GetFriendListIds.
- Expose the permission level of the port instance.
- InvalidUUID => InvalidArgument

* friend: add all cmds as commments

* add Friend structure layout

* Rename FriendErr to FriendError

* Accurately implement INotificationService

* Fix singleton lock of NotificationEventHandler

* Address comments

* Add comments for IDaemonSuspendSessionService cmds

* Explicitly define the Charset when needed

Also make "Nickname" a string

* Address gdk's comments

Ryujinx.HLE/FileSystem/SaveHelper.cs

@@ -32,7 +32,7 @@ namespace Ryujinx.HLE.FileSystem
                 currentTitleId = context.Process.TitleId;
             }
 
-            string saveAccount = saveMetaData.UserId.IsZero() ? "savecommon" : saveMetaData.UserId.ToString();
+            string saveAccount = saveMetaData.UserId.IsNull ? "savecommon" : saveMetaData.UserId.ToString();
 
             string savePath = Path.Combine(baseSavePath,
                 saveMetaData.SaveId.ToString("x16"),