Please add C++20 module support

[Chessom]

Recently, I have used cereal in my own project grouped by C++20 modules. It felt really hard to deal with the header compatibility. I had to wrap the classes and functions by hand. As a result, I think it would be great for library users to have supports for C++modules instead of wrapping those things themselves. Thank you!

[Cheaterdev]

Header unit helps a lot here.

[kelteseth]

We now track cereal modules support progress at https://arewemodulesyet.org/ . Feel free to create a PR if the status changes at https://github.com/kelteseth/arewemodulesyet/ 😊

[redchairman]

您的邮件我已收到，谢谢合作！