Unify the computation of digests and challenges in different folding schemes (#94)

* Remove the trait bound `C::BaseField: PrimeField` for better DX

* Methods in `TranscriptVar` now exactly matches the ones in `Transcript`

* Add `ProtoGalaxyTranscriptVar` and `CommittedInstanceVar` for protogalaxy

* betas are unnecessary in "plain" (incoming) instances

* Absorb the result of `get_challenge_nbits` as well

* `ProtoGalaxyTranscript` now allows absorbing mulitple instances

* Always return `Result<(), SynthesisError>` in `ProtoGalaxyTranscriptVar`

* Impl `Transcript{Var}` for `PoseidonSponge{Var}` directly and remove `PoseidonTranscript{Var}`

* `Transcript::absorb_point` doesn't need to return `Error`

* Add `AbsorbNonNative` trait for hashing non-native values

Note that now `absorb_point` only supports hashing points whose BaseField is equal to the sponge's field

* More efficient `TranscriptVar::absorb_point` by securely removing `is_inf`

* Use `sponge` and `transcript` consistently

* Clarify the usage of `AbsorbNonNative{Gadget}`

* Generic `sponge` and `transcript` params

* Avoid unstable `associated_type_bounds`

* Reuse `sponge` in hypernova

* Clean up redundant imports

* Remove unstable code

* Clarify the usage of `absorb_point` and `absorb_nonnative`

folding-schemes/src/folding/nova/decider_eth.rs

@@ -317,13 +317,11 @@ fn point2_to_eth_format(p: ark_bn254::G2Affine) -> Result<Vec<u8>, Error> {
 
 #[cfg(test)]
 pub mod tests {
-    use ark_bn254::{constraints::GVar, Bn254, Fr, G1Projective as Projective};
-    use ark_groth16::Groth16;
+    use ark_bn254::{constraints::GVar, Fr, G1Projective as Projective};
     use ark_grumpkin::{constraints::GVar as GVar2, Projective as Projective2};
     use std::time::Instant;
 
     use super::*;
-    use crate::commitment::kzg::KZG;
     use crate::commitment::pedersen::Pedersen;
     use crate::folding::nova::PreprocessorParam;
     use crate::frontend::tests::CubicFCircuit;