Implement support for first order predicates in the backend.
Now a statement template can have a predicate hash or a wildcard.
## predicate <-> predicate hash constraints
To build the custom predicate table we need to calculate the custom predicate batch id, which uses the serialization of the statement templates before normalization. This serialization uses the predicate hash when the template uses a predicate (instead of a wildcard). Then in normalization we recalculate the predicate hash if it was a Batch Self.
This means that the relation between hash and predicate must be checked before and after normalization when the template is not using a wildcard. How this is achieved:
- Before normalization: the constructor of StatementTmplTarget forces that if we keep a predicate, it's hash must be equal to the pred_hash when the template has a predicate (and not a wildcard)
- After normalization: the predicate hash is calculated in the normalization and replaced in the case of the template using a predicate and it being a BatchSelf. If it was a predicate but not batch self, the old value was used which was constrained via the constructor.
See `CircuitBuilder::add_virtual_statement_tmpl` and `normalize_st_tmpl_circuit`
## Wildcard predicate resolution
It is done via `make_predicate_from_template_circuit` and is fairly simple as it's contains similar logic to `make_statement_arg_from_template_circuit` but simpler.
- middleware:
- Add `Statement::Intro`
- Add `SignedBy` native predicate and operation. The signature is auxiliary data to the operation
- Rename `PodSigner` to `Signer` with a new API (just for signing `RawValue`)
- Removed `NewEntry` operation. Use `ContainsFromEntries` instead
- Remove `KEY_SIGNER` and `KEY_TYPE` which are no longer used
- Merge `RecursivePod` and `Pod` traits
- Change the `Pod::deserialize_data` method to use `Self` instead of `Box<dyn Pod>`
- Extend `Pod` trait with these methods:
- `is_main`: when the pod is Main, in a (recursive) verification its vk will be checked to exist in the vd_set but not if it's intro pod
- `is_mock`: skip some verifications in the recursive mock MainPod verification
- `verifier_data_hash`
- `pod_id` renamed to `statements_hash`
- AnchoredKeys are now a pair of dictionary root and key
- Entry statements are now defined as Contains with literal arguments
- Operations that take Entries now use Contains statements with literal arguments
- frontend:
- Rename `SignedPod` to `SignedDict` (which now contains the dict, public key and signature, and can still `verify(self)`ed)
- The `SignedDict` keeps the method `get_statement` for convenience but now it returns a `Contains` statement that proves the existence of the key in the dict
- The `MainPodBuilder` automatically inserts a `Contains` statement when an operation is added that uses an entry as argument that was not yet "opened".
- Removed the `literal` methods from the `MainPodBuilder` that were loading literals to anchored keys: that was no longer needed after we introduced literal arguments
- backend
- Only verify inclusion of the verifying key into the vd_set if the pod is MainPod. A pod is not MainPod if the first statement is Intro.
- Reject intro pods that have non-intro statements
- Empty pod now returns an intro statement
- Don't insert a type statement automatically in MainPod and MockMainPod. We get rid of the type entry.
- Implement `SignedBy` operation, which uses the muxed table to store signature verifications
- Rename `PodId` to `statements_hash` or `sts_hash` for short. Now this is only used as a hash of the statements for the circuits public inputs.
- Refactor normalization of `self` statements:
- Before: replace values that contain `SELF` by the given pod_id
- After: place the verifying key hash into the Intro predicates
