Iterable validataclasses and to_dict() reimplementation

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As of now, the only way to iterate over a validataclass instance is using the to_dict() method from the ValidataclassMixin or dataclasses.asdict() (which is used internally by the to_dict() method).

There is a lot of problems that bug me about this solution, for example, since to_dict() only uses asdict(), there is no simple way to alter how the dict representation of nested dataclasses looks like (you cannot simply override to_dict() in the inner dataclass, since it's not used by asdict()). Also, to_dict() filters UnsetValue (and optionally None when #39 is implemented) from the dictionary, but only on the top level, not in nested dictionaries.

Because of this, I want to reimplement to_dict() without using asdict(), so that it recursively calls to_dict() for inner dataclasses.

Besides a usable dict representation, I'd also like to make validataclasses themselves iterable, so you can avoid the conversion to a dictionary if you just want to iterate over the fields. It would also be nice to be able to iterate over the field metadata (i.e. iterate over all fields, so you can access field validators, defaults and other metadata).

This issue is a bit more complex and will introduce breaking changes, and I see it as a necessary part of the 1.0.0 update (though it could be released earlier, maybe with some backwards compatibility measurements that will be dropped in 1.0.0).