supernova: restructure modules

[slumber]

New folding private module, containing NIFS impls.
Besides, rename multifold -> cyclefold (modules only) to avoid confusion for future readers. We might as well add some doc comments on both NIMFS and cyclefold::multifold gadget explaining that those should not be confused with HyperNova for CCS.

new dir tree:

.
├── Cargo.toml
├── README.md
├── rustfmt.toml
├── src
│   ├── absorb.rs
│   ├── circuits
│   │   ├── mod.rs
│   │   └── nova
│   │       ├── mod.rs
│   │       ├── pcd
│   │       │   ├── augmented.rs
│   │       │   ├── compression
│   │       │   │   ├── commitment_utils.rs
│   │       │   │   ├── conversion.rs
│   │       │   │   ├── error.rs
│   │       │   │   └── mod.rs
│   │       │   └── mod.rs
│   │       ├── public_params.rs
│   │       └── sequential
│   │           ├── augmented.rs
│   │           └── mod.rs
│   ├── commitment.rs
│   ├── folding
│   │   ├── hypernova
│   │   │   ├── ml_sumcheck
│   │   │   │   ├── data_structures.rs
│   │   │   │   ├── mod.rs
│   │   │   │   ├── protocol
│   │   │   │   │   ├── mod.rs
│   │   │   │   │   ├── prover.rs
│   │   │   │   │   └── verifier.rs
│   │   │   │   └── tests.rs
│   │   │   └── mod.rs
│   │   ├── mod.rs
│   │   └── nova
│   │       ├── cyclefold
│   │       │   ├── mod.rs
│   │       │   ├── nimfs
│   │       │   │   ├── mod.rs
│   │       │   │   └── relaxed.rs
│   │       │   └── secondary
│   │       │       └── mod.rs
│   │       ├── mod.rs
│   │       └── nifs.rs
│   ├── gadgets
│   │   ├── cyclefold
│   │   │   ├── mod.rs
│   │   │   ├── primary.rs
│   │   │   └── secondary.rs
│   │   ├── mod.rs
│   │   └── nonnative
│   │       ├── mod.rs
│   │       └── short_weierstrass.rs
│   ├── lib.rs
│   ├── provider
│   │   ├── mod.rs
│   │   ├── pedersen.rs
│   │   └── poseidon.rs
│   ├── r1cs
│   │   ├── mod.rs
│   │   └── sparse.rs
│   ├── test_utils
│   │   └── mod.rs
│   └── utils.rs
└── supernova-benches
    ├── benches
    │   └── recursive-snark.rs
    └── Cargo.toml

[tess-eract]

this looks great to me - should improve readability by a good deal.

one note: the compression module will soon have another layer or two of submodules, e.g. circuits for secondary-circuit compression, conversion operations between arkworks and halo2, etc. it might make sense to move this to a top-level module, since it isn't obviously conceptually related to circuits::nova. i put it in its current location originally because that's where the PCD data types are defined, and its implementation is specific to PCD. it's probably more readable to just make a doc-comment at the top level of the module that notes that the compression implementation is PCD-only.

[slumber]

this looks great to me - should improve readability by a good deal.

one note: the compression module will soon have another layer or two of submodules, e.g. circuits for secondary-circuit compression, conversion operations between arkworks and halo2, etc. it might make sense to move this to a top-level module, since it isn't obviously conceptually related to circuits::nova. i put it in its current location originally because that's where the PCD data types are defined, and its implementation is specific to PCD. it's probably more readable to just make a doc-comment at the top level of the module that notes that the compression implementation is PCD-only.

On the contrary: it looks well structured to me that you can compress nova::pcd proof, hence there's this module.

[danielmarinq]

This looks great