Add RecursiveCircuit (#204)

* Add RecursiveCircuit

The RecursiveCircuit verifies N proofs of itself (N=arity), together with
the logic defined at the InnerCircuit (in our case, used for the
MainPodCircuit logic).

The arity defines the maximum amount of proofs of itself that the
RecursiveCircuit verifies. When arity>1, using the RecursiveCircuit has the
shape of a tree of the same arity.

                     π_root
                       ▲
               ┌───────┴────────┐
               │RecursiveCircuit│
               └─▲───▲───▲────▲─┘
         ┌───────┘  ┌┘   └┐   └──────┐
         │π''_1     │ ... │     π''_N│
┌────────┴───────┐ ┌┴┐┌─┐┌┴┐ ┌───────┴────────┐
│RecursiveCircuit│ │.││.││.│ │RecursiveCircuit│
└──▲─────────▲───┘ └─┘└─┘└─┘ └──▲─────────▲───┘
   │         │                  │         │
  π_1  ...  π_N               π'_1 ...  π'_N

where
N: arity of the RecursiveCircuit
π_i: plonky2 proof of the RecursiveCircuit

* add different inner-circuits in the test, reusing the same verifier_data; polish recursion interfaces

* add InnerCircuit::Params

* rm non_base_node

* WIP refactor RecursiveCircuit

* wip. change approach on verifier_data of circuits used in recursivecircuit. arity=1 works

* recursion works fine without registering verifierdatas as publicinputs

* add hashing of verifier_data (out & in-circuit) methods with test

* connect previous and current verifier_datas hashes

* polish

* add InnerCircuit's public_inputs to the RecursiveCircuit

* extend recursive test

* polish & review suggestions

src/backends/plonky2/mod.rs

@@ -4,6 +4,7 @@ mod error;
 pub mod mainpod;
 pub mod mock;
 pub mod primitives;
+pub mod recursion;
 pub mod signedpod;
 
 pub use error::*;