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

Cargo.toml

@@ -23,6 +23,7 @@ espresso_transcript = {git="https://github.com/EspressoSystems/hyperplonk", pack
 [dev-dependencies]
 ark-pallas = {version="0.4.0", features=["r1cs"]}
 ark-vesta = {version="0.4.0"}
+ark-crypto-primitives = { version = "^0.4.0", default-features = false, features = ["crh"] }
 
 [features]
 default = ["parallel"]