Fix Nova multi-elements state (#73)

* Fix Nova multi-elements state

In the AugmentedFCircuit the default value for the state when no input
is provided was `vec![F::zero()]`, which defaults to length `1`. So when
having more than 1 element in the state, before even starting to fold,
the circuit was already already failing.

Additionally this commit adds an example for a circuit with a state of 5
elements.

* abstract 'nova_setup' helper to avoid code duplication in examples

* update example naming to 'MultiInputs'

* rename nova_setup -> test_nova_setup to make it more explicit

folding-schemes/src/folding/nova/circuits.rs

@@ -310,10 +310,14 @@ where
             Ok(self.i.unwrap_or_else(CF1::<C1>::zero))
         })?;
         let z_0 = Vec::<FpVar<CF1<C1>>>::new_witness(cs.clone(), || {
-            Ok(self.z_0.unwrap_or(vec![CF1::<C1>::zero()]))
+            Ok(self
+                .z_0
+                .unwrap_or(vec![CF1::<C1>::zero(); self.F.state_len()]))
         })?;
         let z_i = Vec::<FpVar<CF1<C1>>>::new_witness(cs.clone(), || {
-            Ok(self.z_i.unwrap_or(vec![CF1::<C1>::zero()]))
+            Ok(self
+                .z_i
+                .unwrap_or(vec![CF1::<C1>::zero(); self.F.state_len()]))
         })?;
 
         let u_dummy_native = CommittedInstance::<C1>::dummy(1);