Split Params into base and developer-defined (#458)

I thought it would be nice to have a Predicate for the typed value so that the developer can work with predicates as values comfortably.  Then I noticed that hashing a predicate required `Params` which would have been annoying for converting a `TypedValue::Predicate` to `RawValue` and this led to a small refactor over how `Params` work.

We already had some fields in the `Params` struct that determine compatibility between encoded data.  They can be seen as determining a kind of ABI compatibility.  In general it's better if those parameters don't change so that different circuit configurations can still verify proofs from each other.  So I decided to force those parameters to be constant in the code base and not allow the user of our library to change them.  Many field element serialization/deserialization functions in our code depended on those parameters, and since now they are constant many functions get rid of the `Params` argument, which simplifies the code.  This includes the serialization of a `Predicate` which was required to calculate its hash.

src/lang/frontend_ast_lower.rs

@@ -326,10 +326,10 @@ impl<'a> Lowerer<'a> {
         batches: Option<&PredicateBatches>,
     ) -> Result<MWStatementTmpl, LoweringError> {
         // Enforce argument count limit for request statements
-        if stmt.args.len() > self.params.max_statement_args {
+        if stmt.args.len() > Params::max_statement_args() {
             return Err(LoweringError::TooManyStatementArgs {
                 count: stmt.args.len(),
-                max: self.params.max_statement_args,
+                max: Params::max_statement_args(),
             });
         }
 
@@ -446,6 +446,7 @@ impl<'a> Lowerer<'a> {
             let result = frontend_ast_split::split_predicate_if_needed(pred, self.params)?;
             split_results.push(result);
         }
+
         Ok(split_results)
     }
 }
@@ -676,7 +677,8 @@ mod tests {
         "#;
 
         let params = Params::default();
-        parse_validate_and_lower(input, &params).unwrap();
+        let result = parse_validate_and_lower(input, &params);
+        assert!(result.is_ok());
     }
 
     #[test]