stlab/concurrency/future.hpp won't compile with C++20

Hello,
After updating my local Visual Studio, my program didn't compile anymore. It crashed on the usage of `std::result_of` and `std::result_of_t` in the file stlab/concurrency/future.hpp. Looking into it, I found out that this type is deprecated in C++17, and removed from C++20. I fixed the problem by replacing `std::result_of_t<F(arg)>` by `std::invoke_result_t<F, arg>`.