Add table multiplexer (and use it for container, custom pred & PublicKeyOf ops) (#376)

- Extend the `Flattenable` trait to include a `size` method that returns the number of `Target`s the type requires.  This is used in the table to figure out the max length of an array that must fit all entry types.
- Move the circuit methods to precalculate hash states and do hashes started from a precomputed state to a new module
- Introduce `MuxTableTarget` which allows easy multiplexing of tables where each sub-table may have entries of different lengths.  The table access is done via hashing + unhashing automatically (via use of a generator)
- Use the `MuxTableTarget` to access merkle tree claims and custom predicate verification, which where previously in different tables and accessed with independent random accesses each
- Move the public key derivation for the PublicKeyOf operation check to the same multiplexed table.  Now we can choose how many of those operations a circuit supports.

Resolve https://github.com/0xPARC/pod2/issues/357
Resolve https://github.com/0xPARC/pod2/issues/361

src/backends/plonky2/mock/mainpod.rs

@@ -14,7 +14,7 @@ use crate::{
         mainpod::{
             calculate_id, extract_merkle_proofs, layout_statements,
             process_private_statements_operations, process_public_statements_operations, Operation,
-            Statement,
+            OperationAux, Statement,
         },
         mock::emptypod::MockEmptyPod,
         primitives::merkletree::MerkleClaimAndProof,
@@ -172,14 +172,15 @@ impl MockMainPod {
 
     pub fn new(params: &Params, inputs: MainPodInputs) -> Result<Self> {
         let (statements, public_statements) = layout_statements(params, true, &inputs)?;
+        let mut aux_list = vec![OperationAux::None; params.max_priv_statements()];
         // Extract Merkle proofs and pad.
-        let merkle_proofs = extract_merkle_proofs(params, inputs.operations, inputs.statements)?;
+        let merkle_proofs =
+            extract_merkle_proofs(params, &mut aux_list, inputs.operations, inputs.statements)?;
 
         let operations = process_private_statements_operations(
             params,
             &statements,
-            &merkle_proofs,
-            None,
+            &aux_list,
             inputs.operations,
         )?;
         let operations = process_public_statements_operations(params, &statements, operations)?;