(Multilevel) Scheme CRTP concept

[sandro-elsweijer]

Describe your changes here:
Draft of a new scheme concept using CRTP. This gets rid of all vtables inside our scheme and implements compile-time polymorphism. Should make all element specific functions faster. This concept also shows, that a templated multilevel conversion scheme is possible.
Drawback of this design is, that every scheme function has to be defined not only in the base scheme, but also in the overall scheme, which holds the individual eclass schemes. This is necessary to hide the CRTP to the user. Otherwise the user would need to use the std::visit function and I want to avoid that:
std::visit is everything wrong with modern C++

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