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/backends/plonky2/mainpod/statement.rs

@@ -31,7 +31,7 @@ impl Statement {
 
 impl ToFields for Statement {
     fn to_fields(&self, params: &Params) -> Vec<middleware::F> {
-        let mut fields = self.0.to_fields(params);
+        let mut fields = self.0.hash(params).to_fields(params);
         fields.extend(
             self.1
                 .iter()