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/lang/grammar.pest

@@ -15,8 +15,6 @@ identifier = @{ !("private") ~ (ASCII_ALPHA | "_") ~ (ASCII_ALPHANUMERIC | "_")*
 
 private_kw = { "private:" }
 
-self_keyword = @{ "SELF" }
-
 // Define wildcard names (start with '?')
 wildcard = @{ "?" ~ identifier }
 
@@ -66,10 +64,8 @@ literal_value = {
     literal_array |
     literal_bool |
     literal_raw |
-    literal_pod_id |
     literal_string |
-    literal_int |
-    self_keyword
+    literal_int
 }
 
 // Primitive literal types
@@ -81,7 +77,6 @@ literal_bool = @{ "true" | "false" }
 hash_hex = @{ "0x" ~ (ASCII_HEX_DIGIT ~ ASCII_HEX_DIGIT){32} }
 
 literal_raw = { "Raw" ~ "(" ~ hash_hex ~ ")" }
-literal_pod_id = { hash_hex }
 
 // String literal parsing based on https://pest.rs/book/examples/json.html
 literal_string = ${ "\"" ~ inner ~ "\"" } // Compound atomic string rule
@@ -114,7 +109,6 @@ test_wildcard = { SOI ~ wildcard ~ EOI }
 test_literal_int = { SOI ~ literal_int ~ EOI }
 test_hash_hex = { SOI ~ hash_hex ~ EOI } 
 test_literal_raw = { SOI ~ literal_raw ~ EOI }
-test_literal_pod_id = { SOI ~ literal_pod_id ~ EOI }
 test_literal_value = { SOI ~ literal_value ~ EOI }
 test_statement = { SOI ~ statement ~ EOI }
 test_custom_predicate_def = { SOI ~ custom_predicate_def ~ EOI }