Support defining Union inside a dataclass

[czgdp1807]

No description provided.

[certik]

Perfect! While at this, given the new StructStaticMember, can you also please add a test for:

@dataclass
class A:
    @dataclass
    class B(Union):
        x: i32
        y: i64
    b: B
    c: i32

Essentially a "struct in struct"? We will need it too sooner or later, so now is the time to make sure it works.

[czgdp1807]

Done. Please let me know if there is anything else to do here.

[certik]

I think this is fine. I haven't checked the verify() changes in detail, hopefully they are correct.