Optimize for zero-allocation creating COMB Guid

[skarllot]

• Optimize code for zero-allocation on heap memory when creating COMB Guid;
• Support .NET Standard 2.1 and .NET 5.0;
• Support nullable reference types.

[skarllot]

I've noted, when benchmarking, that the NuGet package from nuget.org was built using Debug configuration instead of Release.

[richardtallent]

Hi! This looks like a great little update. I'm traveling right now, but will review in more detail as soon as I can.

[richardtallent]

Pretty sure I have it ready for release now. Will deploy to nuget when I'm back home later this week. Thanks!