`@autofields` does not take definition order into account

Consider this:

```python
from pyfields import autofields, get_fields

@autofields
class Foo:
    c: int
    b: str = "hello"

get_fields(Foo)  # <-- (<NativeField: Foo.b>, <NativeField: Foo.c>)
```

So the order in which the fields are returned is wrong.


