No Pod IDs (#394)

- 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

src/middleware/error.rs

@@ -3,7 +3,7 @@
 use std::{backtrace::Backtrace, fmt::Debug};
 
 use crate::middleware::{
-    CustomPredicate, Key, Operation, PodId, Predicate, Statement, StatementArg, StatementTmplArg,
+    CustomPredicate, Hash, Key, Operation, Predicate, Statement, StatementArg, StatementTmplArg,
     Value, Wildcard,
 };
 
@@ -24,7 +24,7 @@ pub enum MiddlewareInnerError {
     #[error("{0} should be assigned the value {1} but has previously been assigned {2}")]
     InvalidWildcardAssignment(Wildcard, Value, Value),
     #[error("{0} matches POD ID {1}, yet the template key {2} does not match {3}")]
-    MismatchedAnchoredKeyInStatementTmplArg(Wildcard, PodId, Key, Key),
+    MismatchedAnchoredKeyInStatementTmplArg(Wildcard, Hash, Key, Key),
     #[error("{0} does not match against {1}")]
     MismatchedStatementTmplArg(StatementTmplArg, StatementArg),
     #[error("Expected a statement of type {0}, got {1}")]
@@ -90,14 +90,14 @@ impl Error {
         new!(InvalidWildcardAssignment(wildcard, value, prev_value))
     }
     pub(crate) fn mismatched_anchored_key_in_statement_tmpl_arg(
-        pod_id_wildcard: Wildcard,
-        pod_id: PodId,
+        root_wildcard: Wildcard,
+        root: Hash,
         key_tmpl: Key,
         key: Key,
     ) -> Self {
         new!(MismatchedAnchoredKeyInStatementTmplArg(
-            pod_id_wildcard,
-            pod_id,
+            root_wildcard,
+            root,
             key_tmpl,
             key
         ))