Compilation failures in C++20 due to removal of std::result_of

[IanLilleyT]

I'm using asyncplusplus from a C++20 project and it's failing to compile because std::result_of was removed in C++20. I worked around this problem by replacing std::result_of with std::invoke_result in my fork. You can see the changes here: https://github.com/IanLilleyT/asyncplusplus/commit/1a9cbc504eb675e42524c44f9ecf46973574b257. I opened an issue instead of a PR because I'm not sure if this library wants to include C++17 code in its headers, even if it's behind a macro. Let me know if I should turn this into a PR or not.

The cmake code / comment is partly taken from: https://stackoverflow.com/a/60890947

Thanks for reading!

[Amanieu]

Sure! I'm happy to accept a PR for this.

[IanLilleyT]

Thanks for the super fast turnaround!