Make eantic::renf_elem_class work from Python

[saraedum]

Currently, cppyy.gbl.eantic.renf_elem_class is not functional when called from Python. (See e.g. the special handling of e-antic in flatsurf's surfaces.py.)

At the moment, we work around this problem by exposing a factory function renf_elem that creates a renf_elem_class. However, this has limitations, e.g., when the type is coming from elsewhere say when it is flatsurf::Vector<eantic::renf_elem_class>::Coordinate.

It would be great if we could wrap the actual constructor of renf_elem_class instead so that the type can be used transparently.

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Dependencies

• We cannot remove make_renf_elem_class until https://bitbucket.org/wlav/cppyy/issues/376/stack-overflow-when-calling-private has been resolved.

[saraedum]

While the constructor can now be called without crashing, we are still seeing the wrong output sometimes; essentially due to wlav/cppyy#92 when calling with SageMath objects such as Rational.