Add _eval_simplify helper for AlgebraicNumber

fixes #4401
> Sorry...I'm not sollowing the logic. RootOf

It's only import statement, to filter out such "unevaluated" RootOf's.

> will only return algebraic numbers in the case of quadratics ...

In fact, for any case.  But here, we want only cases when RootOf can be evaluated to some expression with radicals.

> But can't you have algebraic numbers from higher polynomials?

Sure.  E.g. if we can factorize the above cases from such polynomial...