C++20 support on Windows

[godefv]

With GCC 10.2 on linux, I can compile with C++20. It seems to have kept backward compatibility, despite the C++20 standard not requiring it.
With Clang or VS2019 on Windows 10, compilation fails. There a couple of other issues in addition to result_of.

You can check here (godefv/Cinder@f33b943) what I have done to successfully compile with C++20. This commit is on top of this PR.

(see PR #2133 for C++17 with VS2019)