Abstract signature gadget into InnerCircuit. So that PODs just need to
implement the InnerCircuit and then plug and recurse.

Now the number of `(signature proof OR recursive proof) AND ...`
is defined by a parameter `N`.

                   p_root
                   ▲
                   │
               ┌────────┐
               │   F    │
               └────────┘
                ▲ ▲  ▲ ▲
              ┌─┘ │  │ └─┐
         ┌────┘ ┌─┘  └┐  └───┐
         │      │ ... │      │
     ┌────────┐┌┴┐┌─┐┌┴┐ ┌────────┐
     │   F    ││.││.││.│ │   F    │
     └────────┘└─┘└─┘└─┘ └────────┘
     ▲ ▲  ▲  ▲            ▲ ▲  ▲  ▲
   ┌─┘ │  └┐ └─┐        ┌─┘┌┘  └┐ └┐
   │   │   │   │        │  │    │  │
  p_1 p_2 ... p_n     p'_1 p'_2... p'_n

where each p_i is either
   - signature verification
   - recursive plonky2 proof (proof that verifies previous proof)
           (generated by `RecursiveCircuit::prove_step` method)
in other words, each p_i is checking:
  `(signature proof OR recursive proof)`

So for example, by setting `N=2`, we have a binary-tree as we had
in the previous commit, where at each 'node' it is verifying
`(signature proof OR recursive proof) AND (signature proof OR recursive proof)`

By setting `N=3`, each node verifies
`(signature proof OR recursive proof) AND (signature proof OR recursive proof) AND (signature proof OR recursive proof)`

Each node of the tree is verifying:
`((verify left sig OR verify left proof) AND (verify right sig OR verify right proof))`.
and then it generates a new plonky2 proof, which can again be verified
in a node of the next level of the tree.

full binary tree of recursion works