A try at optimization

I found that the program slows down quite a lot when N >= 30
Talking with coworkers, they notice a slowdown too but not that big.
So here I am reading and thinking about each
line of code of my recursive function
trying to spot a really costy operation.

First try is minimizing Copy calls by passing
a pointer for n and verbose and not their value. So that there's
only one spot in the memory that is used for them.