verification/ackermann: Coq 8.5 compatibility
verification/ackermann: Coq 8.5 compatibility

* Use Nat.pow instead of custom pow
* Comment ack_4_2 theorem as it consumes too much time

fishilico/shared,fishilico/shared,fishilico/shared,fishilico/shared,fishilico/shared,fishilico/shared,fishilico/shared