Explicit implemenetation serde::Serialize & Deserialize for types used as keys

[SebastienGllmt]

I'm not sure if there is a good way to codegen this, but the current codegen logic will not explicitly generate serde::Serialize & serde::de::Deserialize traits for types even if they are used as keys in maps which causes a crash on to_json calls

For example, see https://github.com/dcSpark/cardano-multiplatform-lib/issues/115

[SebastienGllmt]

I think this is somewhat related to #96 since this crash is caused by types that have an unclear serialization format such as addresses needing to be converted to string, etc.