Support existential types

[sviperll]

Allow definitions like this:

interface SomeVisitor<T, R> {
    <U extends Callable<T> & Serializable> R callable(U callable);
    R immediate(T value);
}

So we can write

int eval(SomeValue<Integer> value) {
    return value.accept(new SomeVisitor<Integer, Integer>() {
        <U extends Callable<Integer> & Serializable> Integer callable(U callable) {
            return callable.call();
        }
        Integer immediate(Integer value) {
            return value;
        }
    });
}

This will pave the way to GADT support #15