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/mod.rs

@@ -572,10 +572,7 @@ mod tests {
             max_input_pods: 3,
             max_statements: 31,
             max_public_statements: 10,
-            max_statement_args: 6,
             max_operation_args: 5,
-            max_custom_predicate_arity: 5,
-            max_custom_batch_size: 5,
             max_custom_predicate_wildcards: 12,
             ..Default::default()
         };