support polymorphism

given dataclasses like:

```python
@dataclass
class A:
literal: StrLiteral('a')

@dataclass
class B:
literal: StrLiteral('b')

@dataclass
class C:
a_or_b: Union[A, B]
```

`C.to_dict({'literal': 'a'})` and `C.to_dict({'literal': 'b'})` should instantiate the right subclass for `a_or_b`.

voluptuous does this by checking all of the possible subschemas and picking the one that matches, or raises an exception if none of them match. we may want to implement something similar in `to_dict`

another approach would be to leverage json schema's `if/else` construct to specify different subschemas based on a switch field.