feat(core): Support serialization of opaque objects

[potatomashed]

Serialization/deserialization support for mlc.Opaque. Part of #73.

For any MLC object, it now supports:

• .json(...) method adds argument fn_opaque_serialize, which allows users to supply a customized serialization function;
• .from_json(...) adds argument fn_opaque_deserialize, which allows users to supply a customized deserialization function.

By default, the serialization and deserialization methods are:

@mlc.Func.register("mlc.Opaque.default.serialize")
def _default_serialize(opaques: list[Any]) -> str:
    return jsonpickle.dumps(list(opaques))

@mlc.Func.register("mlc.Opaque.default.deserialize")
def _default_deserialize(json_str: str) -> list[Any]:
    return jsonpickle.loads(json_str)

jsonpickle is not a perfect library - usually pickle or cloudpickle could be substantially better in terms of feature completeness, but it actually gives a balance between serializability and readability.