Frontend AST for Podlang (#432)

* Basic frontend AST and semantic validation

* Intro statement support

* Simplify validator lifetime

* Fix arity validation

* Lowering and splitting

* Remove legacy processor and use frontend AST by default

* Use builders instead of creating middleware types directly

* Typos/formatting

* Improve error messages when overflowing a batch due to splitting

* Add FromStr implementation for NativePredicate

* Remove 'raw' fields, and switch HashHex representation to byte vector rather than string

* Simpler wrapper types for batch and intro predicate hashes

* Parse secret and public keys to their respective data structures earlier

* More detail around string escape validity

* Simplify native predicate arity handling and move  method to NativePredicate impl

* Store hashes using middleware::Hash, and simplify lowering by using pre-parsed values

* Simplify predicate building

* Formatting

* Better error messages/suggestions for cases where predicate splitting fails

* Formatting

* Clippy fix

* Return error if we get a too-large int

src/frontend/custom.rs

@@ -165,7 +165,7 @@ impl CustomPredicateBatchBuilder {
 
     /// creates the custom predicate from the given input, adds it to the
     /// self.predicates, and returns the index of the created predicate
-    fn predicate(
+    pub fn predicate(
         &mut self,
         name: &str,
         conjunction: bool,