Real root isolation for fmpz_poly

[fredrik-johansson]

Real root isolation for fmpz_poly is a very frequently requested feature; the complex root-finding is often suboptimal for this.

We could simply copy the following functions from e-antic, perhaps renamed to avoid immediately breaking compatibility with e-antic:

https://flatsurf.github.io/e-antic/c_fmpz_poly_extra.html

I don't think the algorithm is the state of the art, but it should be fine as a starting point.

[videlec]

@fredrik-johansson Do not worry about name clashes. Just do what is best for flint. It will be easy to adapt the e-antic release afterwards.