`to_dict()` -> `to_json()` for metadata classes

[d-v-b]

Our Metadata ABC defines to_dict and from_dict, and we use these methods to recursively read and write nested metadata documents to and from JSON. This would be fine if every implementation of Metadata serialized to a JSON object, but dtypes are a case where we have a rich in-memory representation that serializes to a simple string. data_type.to_dict() -> str is pretty confusing in this case, since one would expect to_dict to return a dict.

Since Metadata is a dataclass, someone who wants to recursively turn one into a dict can already use dataclasses.asdict. So maybe we should remove from_dict and to_dict, and instead define from_json and to_json, methods that turn a metadata model into a JSON-serializable form. This would make the job of these methods more obvious I think, and remove the confusing situation where to_dict doesn't produce a dict.