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).
I thought it would be nice to have a Predicate for the typed value so that the developer can work with predicates as values comfortably. Then I noticed that hashing a predicate required `Params` which would have been annoying for converting a `TypedValue::Predicate` to `RawValue` and this led to a small refactor over how `Params` work.
We already had some fields in the `Params` struct that determine compatibility between encoded data. They can be seen as determining a kind of ABI compatibility. In general it's better if those parameters don't change so that different circuit configurations can still verify proofs from each other. So I decided to force those parameters to be constant in the code base and not allow the user of our library to change them. Many field element serialization/deserialization functions in our code depended on those parameters, and since now they are constant many functions get rid of the `Params` argument, which simplifies the code. This includes the serialization of a `Predicate` which was required to calculate its hash.
* Multi-batch splitting
* Invoke split predicates by name, passing in full argument list
* Reorder batches to prevent failure of forward references where possible
* Rename APIs for clarity
* Simplify example
* Add more docs
* Review updates
* Remove duplicate code
* Comment topological sort algorithm
