Fix ProverKey::serialization_size

[moCello]

Describe what you want implemented
With the change from plonkup back to plonk #684 in the serialization size of the proverkey the poly_num were not adapted correctly, causing the serialization size to be bigger than needed.
Additionally the number of coefficients in a constraint also weren't updated. There used to be 13 coefficients but now that the lookupgate is gone, there are only 12 coefficients in a constraint.

Describe "Why" this is needed
The serialization size was bigger than it needed to be. While the other way round would have much worse implications, this is still to be fixed.

Describe alternatives you've considered
N/A

Additional context
N/A