…t#226)

* Expose the last outputs and number of steps from RecursiveSNARK (microsoft#285)

Both of these data are easily accessible, and could be very useful
to clients:
* Exposing the last outputs allows us to get the current state of
  the computation on the prover side without wasting energy
  recomputing it
* Exposing the number of steps makes it easier to eventually pass
  `num_steps` into `CompressedSNARK::verify`

* Improve error handling (microsoft#286)

* When a function already returns a `Result`, propagate errors
  instead of panicking with `expect`
* For `NovaError::SynthesisError`, retain information about the
  original bellpepper error. Since `NovaError` implements `Clone`
  but bellpepper's `SynthesisError` does not, we keep the error
  information as a `String`.

This commit only fixes low-hanging fruit in lib.rs, for functions
that already return a `Result` and can easily propagate errors just
by replacing `expect(...)` with `?`. There are still many `unwrap()`
calls in functions returning `Result` in other modules, particularly
gadgets. But I don't understand the code well in those parts, and I
suspect some of those `unwrap()`s actually can't fail based on
invariants of the code, so it makes perfect sense to leave them as is.

Co-authored-by: Francois Garillot <[email protected]>

---------

Co-authored-by: Jeb Bearer <[email protected]>

…or Nova & SuperNova (microsoft#285)

* refactor: Refactor codebase to use `Arc` for commitment keys

- Major changes in the handling of the Commitment Key object across several files, moving from direct references to using the Arc (Atomic Reference Counting) mechanism for improved thread-safety and efficient sharing of data.
- Removed the `Abomonation` trait from several structures, such as `ProverKey` and `VerifierKey`.
- Introduced the `SimpleDigestible` trait to VerifierKey in several files, allowing them to be converted into a digestible form for hashing or storage.
- Numerous updates in test cases and instances to reflect the shift from direct Commitment Key references to Arc.
- Refactoring of the `setup` and `prove` methods in multiple files to handle the Arc type for Commitment Key.
- Removal of several `abomonate_with` and `abomonation_skip` attribute annotations.
- Enhancement of error details in multiple `assert` statements.
- Added "rc" feature to the serde dependency in the Cargo.toml file.
- Made comments on minimizing unnecessary data copies and keeping `setup` methods inexpensive.

* refactor: Serialization with Abomonation

- Extended functionality in `lib.rs` with new `PublicParams` structure, efficient abomonationwith `FlatPublicParams`,
- Updated `mod.rs` for conditional feature `abomonate`, added `AuxParams` struct, and implemented conversions between `AuxParams` and `FlatAuxParams`.
- demoed the new use of Abomonation in the minroot example

* refactor: clean up superfluous Abomonation bounds

- Removed Abomonation dependency from several files.

* fix: remove uneeded Abomonation bounds