Replace result_of by invoke_result for C++17 and above #1999

[facug91]

As discussed in #1999, std::result_of is deprecated in C++17 and should be replaced by std::invoke_result. In C++20, std::result_of is removed. What I do in this PR is performing this replacement, when using C++17 or C++20. I used almost the same as in this other PR, from another library fmtlib/fmt#1028.

I also make a minor modification to how std::result_of is use for C++11 and C++14, to comply with how it is recommended (https://en.cppreference.com/w/cpp/types/result_of):

template<class F, class... Args>
std::result_of_t<F&&(Args&&...)> // instead of std::result_of_t<F(Args...)>, which is wrong
  my_invoke(F&& f, Args&&... args) { 
    /* implementation */
}

I ran all tests with gcc7, gcc9 and VS2017 (with C++14, and C++latest).

[davisking]

Awesome, thanks for the fix :)