Add a choiceId for CHOICE

[lamplet]

When i compile an asn.1 like this:
PDU ::= CHOICE { a A, b B }

i want a choiceId in the generated class, so that i can process it with switch/case. And constants CHOICE_ID_A, CHOICE_ID_B like this:

public class PDU implements Serializable
{
    public static final int CHOICE_ID_A = 0;

    public static final int CHOICE_ID_B = 1;

    private int choiceId;

    private A a = null;

    private B b = null;
}

[sfeuerhahn]

I'll put that on my todo list.