solve_univariate: make helper functions private

Following helper functions are now private
* solve_radical
* solve_as_rational
* solve_as_poly
* solve_as_poly_real
* solve_as_poly_complex
* solve_univariate_real_trigonometric_1
* solve_univariate_real_trigonometric_2

All the functionality of these functions can be used by calling
solve_univariate_real or solve_univariate_complex. Calling the above functions
individually doesn't provide any advantage