Serialize PartialSigs as serde tuples

[gakonst]

In #38 we moved to serde, however PartialSignatures (Eval) are now 8 bytes bigger to account for the vector length. We should look into if we can serialize them as tuples (similar to what was done here for Zexe). The extra 8 bytes should not be required, since we know the data format and the lengths.