Allow enum dict keys

[RunOrVeith]

This pull requests allows enums as dict keys.
Since marshmallow does not support anything but string enums by default (see #213 ),
the marshmallow code for enum dict keys also only works with strings.
I've implemented it so that it works for other stuff (e.g. float, int) when using from_json, to_json,
if that is not desired let me know.

This means you can now do:

@dataclass_json
@dataclass(frozen=True)
class DataWithEnumKeys:
    name: str
    enum_dict: Dict[MyEnum, str]


d_keys = DataWithEnumKeys(name="name1", enum_dict={MyEnum.STR1: "str_test",
                                                   MyEnum.INT1: "int_test"})
d_keys.to_json() # '{"name": "name1", "enum_dict": {"str1": "str_test", "1": "int_test"}}'
DataWithEnumKeys.from_json(d_keys.to_json()) == d_keys  # True

@lidatong

[RunOrVeith]

@lidatong

[lidatong]

I don't believe this is necessarily true -- enum names are not necessarily one-to-one with values. There is a decorator @unique that enforces it, but the standard library does allow for it

[lidatong]

Appreciate the contribution again! But I think this is functionality should be supplied via config by the user. Specifically, you can specify the decoder for the field. I purposely limited the supported types, with the intention of only supporting the most common use-cases and the user being able to extend the library otherwise.

While I see the motivation / use-case for Enum keys in dicts, I don't think the library code itself should try to do that decoding -- there's a tiny bit of code overhead with each feature that adds up to the library's maintainability. I'm struggling to read through / follow the library code much more compared to before, and I'm feeling quite strongly to not add any more features / additions, and focus on closing issues / performance / refactoring / simplifying.

I do appreciate your work on this library a lot, and let me know if you think differently.

[ole-tidal]

I would like to have this feature as well. Super handy to have especially when coming from Jackson in java.