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/primitives/signature/circuit.rs

@@ -23,7 +23,7 @@ use plonky2::{
 
 use crate::{
     backends::plonky2::{
-        basetypes::{Proof, C, D},
+        basetypes::{C, D},
         circuits::common::{CircuitBuilderPod, ValueTarget},
         error::Result,
         primitives::signature::{
@@ -31,7 +31,7 @@ use crate::{
         },
     },
     measure_gates_begin, measure_gates_end,
-    middleware::{Hash, RawValue, EMPTY_HASH, EMPTY_VALUE, F, VALUE_SIZE},
+    middleware::{Hash, Proof, RawValue, EMPTY_HASH, EMPTY_VALUE, F, VALUE_SIZE},
 };
 
 lazy_static! {