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/serialization.rs

@@ -19,7 +19,7 @@ use serde::{de, ser, Deserialize, Serialize};
 
 use crate::backends::plonky2::{
     basetypes::{CircuitData, CommonCircuitData, VerifierCircuitData, C, D, F},
-    circuits::{common::LtMaskGenerator, utils::DebugGenerator},
+    circuits::{common::LtMaskGenerator, mux_table::TableGetGenerator, utils::DebugGenerator},
     primitives::ec::{
         bits::ConditionalZeroGenerator,
         curve::PointSquareRootGenerator,
@@ -92,7 +92,6 @@ use plonky2::{
 #[derive(Debug)]
 pub(crate) struct Pod2GeneratorSerializer {}
 
-// TODO: Add pod2 custom generators
 impl WitnessGeneratorSerializer<F, D> for Pod2GeneratorSerializer {
     impl_generator_serializer! {
         Pod2GeneratorSerializer,
@@ -130,7 +129,8 @@ impl WitnessGeneratorSerializer<F, D> for Pod2GeneratorSerializer {
         RecursiveGenerator<1, NNFMulSimple<5, QuinticExtension<F>>>,
         RecursiveGenerator<D, ECAddHomogOffset>,
         RecursiveGenerator<1, ECAddHomogOffset>,
-        ComparisonGenerator<F, D>
+        ComparisonGenerator<F, D>,
+        TableGetGenerator
     }
 }