refactor: code and improve testing accuracy

- removed `bigint` and `constr` from the `conway` module in the `chain_observer`'s `codec.rs` file

mithril-common/src/chain_observer/codec.rs

@@ -1,4 +1,4 @@
-use pallas_primitives::conway::{BigInt, Constr, PlutusData};
+use pallas_primitives::conway::PlutusData;
 use pallas_primitives::ToCanonicalJson;
 
 use super::model::Datum;