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/basetypes.rs

@@ -169,6 +169,12 @@ pub struct Hash(
     pub [F; HASH_SIZE],
 );
 
+impl From<Hash> for HashOut {
+    fn from(hash: Hash) -> HashOut {
+        HashOut { elements: hash.0 }
+    }
+}
+
 impl ToHex for Hash {
     fn encode_hex<T: std::iter::FromIterator<char>>(&self) -> T {
         self.0