Turn each missing feature into an issue, to track it

[tvaneerd]

See feature_tracking.md for (partial?) list of C++20 features

Turn each missing feature into an issue.

Then fix the issues by writing tables :-)

Also, some links to definitive lists of C++20 might help. There is a wikipedia list, but also, somewhere a more formal wg21 list, I think.

[tvaneerd]

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p2131r0.html
https://en.wikipedia.org/wiki/C%2B%2B20
https://en.cppreference.com/w/cpp/compiler_support