Move enum and dataclass registry lookups from `serialization.py` and `casting.py` to `registry.py`

[augray]

There are a lot of checks when looking up registered functions along the lines of "if I didn't find this registered, and it's a dataclass/enum, check the key for dataclass/enum." Instead, it would be better if the registry itself had logic when looking up a type that was either a dataclass/enum if it fell back to use those keys.