Support child class / interface implementations

[kevinresol]

I am not very sure if this make sense for tink_json, but it would be quite useful if supported.

interface Base {}

class Foo implements Base {}
class Bar implements Base {}

function main() {
	var base:Base;
	var s = tink.Json.stringify(base); // store the actual runtime class somehow
	base = tink.Json.parse(s); // get back the correct implementation
}