refactor: replace direct SNARK with batched SNARK

[adr1anh]

The implementations of the batched SNARKs are should be equivalent to the non-batched versions. We can replace the direct implementation with the batched one using a number of instance = 1.

• direct: Conditionally squeeze outer_r challenge in cases where number of instances = 1.
• pre-processing: Move all SumcheckEngine related structs from ppsnark.rs to sumcheck.rs
• Implement RelaxedR1CSSNARK for batched_ppsnark::RelaxedR1CSSNARK and batched::BatchedRelaxedR1CSSNARK using vectors of size 1.

This should help with maintenance going forward as we will only have to maintain 2 SNARKs rather than 4.

[huitseeker]

1. see Refactor pre-processing SNARK #220 for the pre-processing step
2. I think we could simply delete the direct SNARK, as it is unused, and there are no plans for using it.

[adr1anh]

+1 on removing the direct SNARK. it would also allow us to remove a lot of of the SumcheckProof::prove_* and focus on extending the SumcheckEngine approach.