Implement HyperNova's IVC into the FoldingScheme trait (#116)

- implement the IVC `FoldingScheme` trait for HyperNova
- refactor Nova's preprocess logic to make it simplier to use
- add to Decider trait (& Nova's DeciderEth) a preprocess method
- get rid of the `init_nova_ivc_params` and `init_ivc_and_decider_params` methods in `examples` since this is achieved with the `FS::preprocess` & `Decider::preprocess` methods
  - (update the examples code to the simplified interface using
    FS::preprocess & Decider::preprocess)

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

@@ -39,6 +39,7 @@ where
         (w_dummy, u_dummy)
     }
 
+    // notice that this method does not check the commitment correctness
     fn check_instance_relation(
         &self,
         W: &Witness<C>,
@@ -52,6 +53,7 @@ where
         self.check_relation(&Z)
     }
 
+    // notice that this method does not check the commitment correctness
     fn check_relaxed_instance_relation(
         &self,
         W: &Witness<C>,