Fix std::result_of removed in C++20

[Asaaj]

Fix for #530, intended to be as minimal as possible. Further testing is required, but I wanted to get this in for discussion.

[Asaaj]

I wanted a simpler change than #545, which apparently failed. This seems safer to me

[Asaaj]

I've never worked with AppVeyor before, but is it reasonably straightforward to get it building with VS2019? Or is RxCpp not claiming to support that version of VS? I can guess what changes are required in appveyor.yml, but I'm not sure the protocol here.

[Asaaj]

@kirkshoop with C++20 approaching completion on all major compilers, it would be great to get this fix tested and released soon so this library doesn't gate those who want to upgrade (myself included).

[serg06]

@kirkshoop please 🙏 my C++20 project needs this

[serg06]

Update: Here's a quick workaround, before importing rx.hpp have this:

namespace std {
   template <class> struct result_of;
   template <class F, class... TN> struct result_of<F(TN...)>
   {
      using type = std::invoke_result_t<F, TN...>;
   };
}

[serg06]

❤️

[david-alvarez-rosa]

Thanks!!