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

Author: arnaucube

Cargo.toml

@@ -23,6 +23,7 @@ serde = "1.0.219"
 serde_json = "1.0.140"
 base64 = "0.22.1"
 schemars = "1.0.0-alpha.17"
+hashbrown = { version = "0.14.3", default-features = false, features = ["serde"] }
 
 # Uncomment for debugging with https://github.com/ed255/plonky2/ at branch `feat/debug`.  The repo directory needs to be checked out next to the pod2 repo directory.
 # [patch."https://github.com/0xPolygonZero/plonky2"]

src/backends/plonky2/mod.rs

@@ -3,4 +3,5 @@ pub mod circuits;
 pub mod mainpod;
 pub mod mock;
 pub mod primitives;
+pub mod recursion;
 pub mod signedpod;