Fix opa serialization of lists

[HenriChataing]

It may be needed to have opa serialization translate lists to equivalent Json arrays, for example for the implementation of plugins. This has been implemented in OpaSerialize.serialize, but not in the core.

And a bug in OpaSerialize: sometimes, lists are badly translated: OpaSerialize.serialize(["a"]) returns {"hd": "a", "tl": []} OpaSerialize.serialize(["a","b"]) returns {"hd": "a", "tl": ["b"]}

[HenriChataing]

This is due to list types being expanded into {'a hd, list('a) tl}, and is solved by adding test cases to the serialisation of records and rows.