Additional reactive library targeting R3?

[nathanAjacobs]

@TolikPylypchuk Are you open to the idea of adding an additional reactive library targeting R3 (https://github.com/Cysharp/R3) to this project?

If so, I would be willing to contribute a pull request for the implementation.

[TolikPylypchuk]

Hello @nathanAjacobs, thanks for asking! There was a request about R3 integration earlier this year (#130) which I've declined. But, having reviewed it today, I've decided that it's OK to add it as it adds little in terms of complexity, and it will be really similar to integration with Rx.NET.

I'll add it myself (I've already implemented it in a small console project, it took like 5 minutes to do), and then, if you don't mind, you can review whether it looks OK as I don't have any prior experience with R3.

Also, let's move this discussion to issue #130 so that it's not fragmented.