constexpr everything

[DiamonDinoia]

We should constexpr everything available in c++17 as this allows to do part of the computation at compile time when the data is needed.

We should also add macros CPP_20_CONSTEXPR CPP_23_CONSTEXPR and have cmake use the latest available standard in compiling so that even malloc/free can be constexpr in that case. A lot of the planning phase might be executed at compile time when the users bake finufft in their code as a source, this can make a meaningful difference.

This can be done in #482 #483 as it requires some code rewrite anyway.