Commit 1d4369c2 breaks C++20 builds

@cantonios Commit 1d4369c2 breaks compiling with C++20, std::result_of is removed in C++20, I believe the old version using the non-prefixed namespace version was taken care of by header configs to detect this (EIGEN_HAS_STD_RESULT_OF).

You can use std::invoke_result from C++17 onwards.