Support for optional data class parameters, tests for null encoding in data classes

[comp500]

• Added support for optional data class parameters (default arguments) - allows specifying default values per-field rather than per-type
• Improved exception thrown when unknown value/target pair is encountered
• Changed to Any? for parameter decoding to better reflect how decoders can decode to null (though I think it is unchecked at compile-time and doesn't complain with Any)
• Added tests for the above

[valderman]

Very nice!

Just a minor comment: could we use associateWith to build the map passed to callBy´ for readability? That way we get rid of the continue`, we get to use a function with a more specific name instead of general iteration, and we don't need to mutate anything.

[comp500]

Does that allow skipping optional parameters? I guess I could add a filter/mapNotNull before the associateWith but it seems a bit clunky to pass both encodedParameterValue and constructorParameter along after filtering.

[valderman]

You're right, that doesn't work; I didn't consider null parameters. Let's go with this as it is!