Eliminate allocations

[iamcarbon]

What are your thoughts on converting all the classes to readonly structs to eliminate allocations in the library?

I've started a PR to see what this would look like. The API would stay the same although any library depending on things would need to recompile.

This could also be a good time to increase the library to V2 and drop support for .NET 4.0.

[tompazourek]

This looks like a good idea to me. I've actually originally started building the library with structs instead of classes, but now when we have the readonly structs, I think it might be an opportunity to reconsider.

I haven't got much free time right now to checkout the fork and get a closer look, but I'd like to try it out in the following weeks (weekends/evenings). I haven't yet had a chance to play with any readonly structs, and I think it's a good chance to explore it. In general, I am all in favor of immutability, especially when the compiler can help with it and utilize it as well...

Thanks!

[iamcarbon]

I believe this covers everything. All tests continue to pass with no changes.

[tompazourek]

Merged the PR today, sorry it took a while.

I ended up dropping the IRGB interface for now. I've also done some small code formatting cleanup (mostly using expression-bodied members more).

I just published this as version 2.0.0 on NuGet now.

Thanks again for the great work.