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/utils/lagrange_poly.rs

@@ -52,7 +52,7 @@ mod tests {
     use crate::utils::lagrange_poly::compute_lagrange_interpolated_poly;
     use ark_pallas::Fr;
     use ark_poly::{univariate::DensePolynomial, DenseUVPolynomial, Polynomial};
-    use ark_std::{vec::Vec, UniformRand};
+    use ark_std::UniformRand;
     use espresso_subroutines::poly_iop::prelude::PolyIOPErrors;
 
     #[test]