C++20 support

[lbt0116]

The project currently faces compatibility issues with C++20, such as disallowing template parameters in destructors (e.g., ~ConstraintSetTpl()) and the use of non-constexpr types like std::string in constant expressions, which prevent leveraging modern C++20 features. To address this, I propose ensuring full C++20 compliance by updating problematic code, replacing non-compliant patterns, and adding explicit C++20 support in the build system (e.g., CMake flags). This would enable developers to benefit from C++20's improvements like concepts, ranges, and enhanced constexpr, ensuring the library stays modern and developer-friendly.

[ManifoldFR]

Hello, thanks for your interest in aligator!

Could you elaborate on some of these points? I am not familiar with C++20 beyond concepts.
We haven't looked at implementing C++20 features yet, due to both lack of time and lack of return on investment: inherent problems in even several robotics software stacks (looking at you, ROS) limited us to C++14 when we first started. Getting to C++17 was already a difficult negotiation.

We could get onto it with your help and the help of other developers if we could get a report (and a PR down the line) on e.g. noncompliant code and how to fix the CMake build flags.