added custom time based guids

Core/GuidV8.cs

@@ -0,0 +1,40 @@
+using System.Runtime.InteropServices;
+using System.Security.Cryptography;
+
+namespace Just.Core;
+
+public enum GuidV8Entropy { Strong, Weak }
+
+public static class GuidV8
+{
+    [Pure, MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public static Guid NewGuid(GuidV8Entropy entropy = GuidV8Entropy.Strong) => NewGuid(DateTime.UtcNow, entropy);
+
+    [Pure]
+    public static Guid NewGuid(DateTime dateTime, GuidV8Entropy entropy = GuidV8Entropy.Strong)
+    {
+        var timestamp = dateTime.ToBinary() & 0x3FFF_FFFF_FFFF_FFFF;
+
+        Span<byte> ts = stackalloc byte[8];
+        MemoryMarshal.Write(ts, timestamp);
+
+        Span<byte> bytes = stackalloc byte[16];
+
+        ts[4..].CopyTo(bytes[..4]);
+        ts[2..4].CopyTo(bytes[4..6]);
+        ts[..2].CopyTo(bytes[6..8]);
+
+        bytes[7] = (byte)((bytes[7] & 0x0f) | 0x80);
+
+        if (entropy == GuidV8Entropy.Strong)
+        {
+            RandomNumberGenerator.Fill(bytes[8..]);
+        }
+        else
+        {
+            Random.Shared.NextBytes(bytes[8..]);
+        }
+
+        return new Guid(bytes);
+    }
+}