feat (circom): allow to define the step_native in Rust

[vuvoth]

Solve #98

Example

 fn test_custom_code() {
        let r1cs_path = PathBuf::from("./src/frontend/circom/test_folder/cubic_circuit.r1cs");
        let wasm_path =
            PathBuf::from("./src/frontend/circom/test_folder/cubic_circuit_js/cubic_circuit.wasm");

        let mut circom_fcircuit = CircomFCircuit::<Fr>::new((r1cs_path, wasm_path, 1, 0)).unwrap(); // state_len:1, external_inputs_len:0

        circom_fcircuit.set_custom_step_native(Rc::new(|_i, z_i, _external| {
            let z = z_i[0];
            Ok(vec![z * z * z + z + Fr::from(5)])
        }));

        // Allocates z_i1 by using step_native function.
        let z_i = vec![Fr::from(3_u32)];
        let wrapper_circuit = crate::frontend::tests::WrapperCircuit {
            FC: circom_fcircuit.clone(),
            z_i: Some(z_i.clone()),
            z_i1: Some(circom_fcircuit.step_native(0, z_i.clone(), vec![]).unwrap()),
        };

        let cs = ConstraintSystem::<Fr>::new_ref();

        wrapper_circuit.generate_constraints(cs.clone()).unwrap();
        assert!(
            cs.is_satisfied().unwrap(),
            "Constraint system is not satisfied"
        );
    }