Problem: ackermann benchmark is slow
Problem: ackermann benchmark is slow

It clocks at 28-30ms for `3 4 ack` on my laptop.

Solution: provide a pure stack-based implementation of `ack` to compare it with
the one that uses dictionaries (the original
one).

This improves benchmark performance by about 20%.

Both benchmarks were kept to track the difference in the approach.

PumpkinDB/PumpkinDB