Circom external inputs (#91)

* circom: add external_inputs

* adapt new external_inputs interface to the FoldingScheme trait and Nova impl

* adapt examples to new FCircuit external_inputs interface

* add state_len & external_inputs_len params to CircomFCircuit

* add examples/circom_full_flow.rs

* merge the params initializer functions, clippy

* circom: move r1cs reading to FCircuit::new instead of each step

* CI/examples: add circom so it can run the circom_full_flow example

folding-schemes/src/lib.rs

@@ -124,7 +124,7 @@ where
         z_0: Vec<C1::ScalarField>, // initial state
     ) -> Result<Self, Error>;
 
-    fn prove_step(&mut self) -> Result<(), Error>;
+    fn prove_step(&mut self, external_inputs: Vec<C1::ScalarField>) -> Result<(), Error>;
 
     // returns the state at the current step
     fn state(&self) -> Vec<C1::ScalarField>;