The `std::result_of` meta struct is deprecated in C++17 and removed
in C++20. It was still slipping through due to a faulty definition of
`EIGEN_HAS_STD_RESULT_OF`.
Added a new macro `EIGEN_HAS_STD_INVOKE_RESULT` and
`Eigen::internal::invoke_result` implementation with fallback for
pre C++17.
Replaces the `result_of` definition with one based on `std::invoke_result`
for C++17 and higher.
For completeness, added nullary op support for c++03.
Fixes#1850.
a31effc3bc