Deserialize Generics/Collections in Union Types

Bugfix: support for properly deserializing Union types that contain generics (e.g. collection types such as List, Dict, etc.). In general, the following will work now:

deserialize(Union[A[B], C[D]], x)

For @dataclass(frozen=True) class A(Generic[B]) and @dataclass(frozen=True) class C(Generic[D]).