MatrixFree: Unify DoFInfo and ConstraintInfo

[kronbichler]

While reviewing #15141, I realized once more that https://github.com/dealii/dealii/blob/master/include/deal.II/matrix_free/constraint_info.h, i.e., internal::MatrixFreeFunctions::ConstraintInfo duplicates too much information from internal::MatrixFree::DoFInfo, to be usable in the context of multigrid algorithms. As posted in #13519 (review), we should really aim to see this class as an intermediate step towards a possible restructuring in the interfaces, not a long-term solution.

In view of #14968 (that unfortunately seems to be low-priority), this is also a serious performance concern. One possible solution to the performance aspects would be to try if some of the two-level transfer objects (essentially all non-geometric ones that use two DoFHandler objects based on the same triangulation) can use the MatrixFree objects directly and only link the two indices with each other. Now that we have

dealii/include/deal.II/matrix_free/fe_evaluation.h

Lines 2144 to 2145 in aef0f70

	void
	reinit(const std::array<unsigned int, VectorizedArrayType::size()> &cell_ids);

from #13260, this should be rather simple. Of course, we still have the code duplication aspects, that also need to addressed. However, I consider the performance aspects (and the associated memory consumption) even more important.