[RFC] Move to C++20

[bernhardmgruber]

I strongly consider switching LLAMA to C++20 in the next months. There are currently no downstream users of LLAMA that depend on C++17. If you read this and you are one, or have serious concerns, please add a comment here!

The move is mainly motivated by me starting to write my PhD thesis on LLAMA in a few months. And when I sit down and take 6 months to write about LLAMA, I would like to do the best job possible. And with C++20, LLAMA is probably easier to understand and explain, because I could really write down what e.g. a "record reference" or a "proxy reference" or a "blob" is in actual syntax and not just words. Also, it seems like I can do all my benchmarks needed for my PhD with C++20.