- Minor optimization in the Number class: try to reuse mpz objects that are about to be destoryed.
- Minor optimization in the Number class: try to reuse mpz objects that are about to be destoryed.

This particularily improves the performance of the `n.times {}` construct. At each iteration, if possible, the same mpz object will be reused. Previously, a new mpz object was created and destoyed at each iteration, which was a bit costly.

trizen/sidef,trizen/sidef,trizen/sidef,trizen/sidef