Merkle tree for custom predicate batches (#471)

Resolve https://github.com/0xPARC/pod2/issues/466

Now batches are identified by the root of a merkle tree that contains all the predicates (using sequential indices as keys).  This means that the format to identify a custom predicate reference is still a hash + index, but the calculation of the hash is different.
The MainPod circuit now isn't limited by number of batches but instead number of custom predicates; and for each one we verify a merkle proof to verify the batch id.

I've removed a bunch of tests from lang that were testing splitting into multiple batches because there's no longer any need for that.  In a future PR we'll remove the code that handles batch splitting.

Each custom predicate needs 148.2 gates (which is very close to my estimate of 142.7 in https://github.com/0xPARC/pod2/issues/466#issuecomment-3823531286 where I actually made a mistake and considered 5 predicates per batch instead of 4 in the previous Params).

src/lang/frontend_ast_validate.rs

@@ -777,7 +777,7 @@ mod tests {
         )
         .unwrap();
 
-        let batch = CustomPredicateBatch::new(&params, "TestBatch".to_string(), vec![pred]);
+        let batch = CustomPredicateBatch::new("TestBatch".to_string(), vec![pred]);
 
         let batch_id = batch.id().encode_hex::<String>();
         let input = format!(