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

Cargo.toml

@@ -41,7 +41,7 @@ serde_arrays = "0.2.0"
 sha2 = { version = "0.10.9" }
 
 # Uncomment for debugging with https://github.com/ed255/plonky2/ at branch `feat/debug`.  The repo directory needs to be checked out next to the pod2 repo directory.
-# [patch."https://github.com/0xPolygonZero/plonky2"]
+# [patch."https://github.com/0xPARC/plonky2"]
 # plonky2 = { path = "../plonky2/plonky2" }
 
 [dev-dependencies]
@@ -61,3 +61,7 @@ time = []
 examples = []
 disk_cache = ["directories", "minicbor-serde"]
 mem_cache = []
+
+# Uncomment in order to enable debug information in the release builds.  This allows getting panic backtraces with a performance similar to regular release.
+# [profile.release]
+# debug = true