Better support for user accounts (#349)

* Better support for user accounts

* Nits

* Check for invalid ids

Ryujinx.HLE/OsHle/SystemState/UserId.cs

@@ -0,0 +1,76 @@
+using Ryujinx.HLE.OsHle.Utilities;
+using System;
+using System.IO;
+using System.Linq;
+
+namespace Ryujinx.HLE.OsHle.SystemState
+{
+    public struct UserId
+    {
+        public string UserIdHex { get; private set; }
+
+        public byte[] Bytes { get; private set; }
+
+        public UserId(long Low, long High)
+        {
+            if ((Low | High) == 0)
+            {
+                throw new ArgumentException("Zero is not a valid user id!");
+            }
+
+            byte[] Bytes = new byte[16];
+
+            int Index = Bytes.Length;
+
+            void WriteBytes(long Value)
+            {
+                for (int Byte = 0; Byte < 8; Byte++)
+                {
+                    Bytes[--Index] = (byte)(Value >> Byte * 8);
+                }
+            }
+
+            WriteBytes(Low);
+            WriteBytes(High);
+
+            UserIdHex = string.Empty;
+
+            foreach (byte Byte in Bytes)
+            {
+                UserIdHex += Byte.ToString("X2");
+            }
+
+            this.Bytes = Bytes;
+        }
+
+        public UserId(string UserIdHex)
+        {
+            if (UserIdHex == null || UserIdHex.Length != 32 || !UserIdHex.All("0123456789abcdefABCDEF".Contains))
+            {
+                throw new ArgumentException("Invalid user id!", nameof(UserIdHex));
+            }
+
+            if (UserIdHex == "00000000000000000000000000000000")
+            {
+                throw new ArgumentException("Zero is not a valid user id!", nameof(UserIdHex));
+            }
+
+            this.UserIdHex = UserIdHex.ToUpper();
+
+            Bytes = StringUtils.HexToBytes(UserIdHex);
+        }
+
+        internal void Write(BinaryWriter Writer)
+        {
+            for (int Index = Bytes.Length - 1; Index >= 0; Index--)
+            {
+                Writer.Write(Bytes[Index]);
+            }
+        }
+
+        public override string ToString()
+        {
+            return UserIdHex;
+        }
+    }
+}