Use flint-3 for the subproduct trees in the trial division step.

* lib/cm-impl.h (cm_flint_tree_gcd): Declare new function.
* lib/flint.c (cm_flint_tree_gcd): New function.
* lib/nt.c (tree_gcd): Drop function when flint-3 is available.
  (cm_nt_mpz_tree_gcd): Use new function when flint-3 is available.