Potential Update for C++20

[gconeice]

Hi,

I noticed that the file emp-tool/emp-tool/utils/ThreadPool.h is still using std::result_of, which has been discarded in C++20. This can be simply fixed by using std::invoke_result instead.

I can make a pull request if needed.

Thanks!

[gconeice]

Note that you may also need macros to switch between C++ <= 14 and C++ > 14.

[wangxiao1254]

That would be appreciated!