The comparison functor passed to the constraints hierarchy has a
pointer to the `Constrained_triangulation_plus_2` that contains it.
That means the defaulted move constructor and move assignment operator of `Polyline_constraint_hierarchy_2` cannot be used to
implement the move constructor and move assignment operator of
`Constrained_triangulation_plus_2`.
Add a special constructor for `Polyline_constraint_hierarchy_2`, that is
usee to move the hierarchy and reassign the comparison operator.
Laurent Rineau
161a313109