Use predicate hash in statements instead of the literal predicate

Resolve #448 

Previously a predicate was 6 elements.  Now it grows to 8 elements; and the hash is 4 elements.

Some parts of the circuit require only require equality checks with the predicate: that works with the predicate hash.  Other parts require inspecting or working with particular elements in the predicate, those need the preimage of the predicate hash.
Both `StatementTarget` and `StatementTmplTarget` have been updated to include the predicate hash and optionally the predicate.  When the predicate is included, constraints are automatically generated for `pred_hash = hash(pred)`.  We only include the predicate when needed.

src/middleware/custom.rs

@@ -170,6 +170,7 @@ impl ToFields for StatementTmpl {
 
         let mut fields: Vec<F> = self
             .pred
+            .hash(params)
             .to_fields(params)
             .into_iter()
             .chain(self.args.iter().flat_map(|sta| sta.to_fields(params)))