Add decider circuit RelaxedR1CS (#21)

- Add naive decider circuit `RelaxedR1CSGadget`, which in-circuit checks
  that the given z satisfies the given RelaxedR1CS instance
- Add method to relax the R1CS instance
- Add check_relation (for testing only) to R1CS & RelaxedR1CS
- Migrate from own SparseMatrix to use ark_relations::r1cs::Matrix
- Add frontend helper to use arkworks circuits

src/lib.rs

@@ -10,7 +10,9 @@ pub mod transcript;
 use transcript::Transcript;
 pub mod ccs;
 pub mod constants;
+pub mod decider;
 pub mod folding;
+pub mod frontend;
 pub mod pedersen;
 pub mod utils;