Port/hypernova multifolding (#10)

* Port HyperNova's multifolding from https://github.com/privacy-scaling-explorations/multifolding-poc adapting and refactoring some of its methods and structs.

Note: adapted mle.rs methods from dense to sparse repr.

Co-authored-by: George Kadianakis <desnacked@riseup.net>

* HyperNova: move CCS struct outside of LCCCS & CCCS

HyperNova nimfs: move CCS structure outside of LCCCS & CCCS, to avoid
carrying around the whole CCS and duplicating data when is not needed.

Also add feature flags for the folding schemes.

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Co-authored-by: George Kadianakis <desnacked@riseup.net>

src/folding/nova/mod.rs

@@ -1,3 +1,4 @@
+/// Implements the scheme described in [Nova](https://eprint.iacr.org/2021/370.pdf)
 use ark_crypto_primitives::sponge::Absorb;
 use ark_ec::{CurveGroup, Group};
 use ark_std::fmt::Debug;