When including CGAL/Polynomial/CORE_Expr_root_stack.h in a header file
that in turn gets included in multiple .cpp files, code from
CORE_Expr_root_stack.h ends up in multiple object files and causes
linking problems:
multiple definition of `CGAL::POLYNOMIAL::operator<<(std::ostream&, CGAL::POLYNOMIAL::CORE_Expr_root_stack const&)'
multiple definition of `CGAL::POLYNOMIAL::internal::operator*(CORE::BigRat const&, CGAL::POLYNOMIAL::internal::CORE_polynomial const&)'
Work around this issue by declaring both operators inline.