PCF calculator optimizations

[itaybthl]

1. Currently, the calculations always use integers (mpz) for computation, on the assumption that integer arithmetic is fastest. Investigate whether rational number arithmetic (with mpqs) is just as fast.
2. If part 1 is successful, transition to calculating directly on the canonical form, instead of inflating and then computing.
3. If part 2 of this issue is done, consider removing the "mergesort-style" computation, and making a specialized matrix multiplication that makes use of the fact that at every step, 3 out of the 4 matrix elements are either 0 or 1, which should theoretically make computation of each step very simple.

[itaybthl]

Using mpqs directly is slower since they always seek to cancel out the GCD. Same goes for python's native fractions.Fraction. The only way this can work is to either find a different rational numbers library, or implement it from scratch, but keep in mind that sometimes eliminating the GCD can boost performance still.