Integrate recursion into MainPod (#243)

* calculate MainPod id in a dynamic-friendly way

The MainPod id is now calculated with front padding and a fixed size
independent of max_public_statements so that introduction gadgets can be
verified by a MainPod while paying only for the number of statements
they use.  This is because with front padding of none-statements we can
precompute the poseidon state corresponding to absorbing all the padding
statements and only pay constraints for the non-padding statements.

The id is calculated as follows:
`id = hash(serialize(reverse(statements || none-statements)))`

* add time feature and disable timing by default

* apply suggestions from @arnaucube

* link issues in todos

src/backends/plonky2/circuits/signedpod.rs

@@ -83,7 +83,7 @@ pub struct SignedPodVerifyTarget {
     // the KEY_TYPE entry must be the first one
     // the KEY_SIGNER entry must be the second one
     mt_proofs: Vec<MerkleProofExistenceTarget>,
-    signature: SignatureVerifyTarget,
+    pub(crate) signature: SignatureVerifyTarget,
 }
 
 impl SignedPodVerifyTarget {