Standardize pandas metadata for table schema and parquet
Both `to_json(orient='table')` and `to_parquet` write some metadata about the dataframe. It'd be nice to standardize these.

Discrepancies

- Default index names ('index' vs. `'__index_level_d__'`)

We should also add a function for parsing the metadata, and perhaps constructing an empty DataFrame with the correct index, types, and column names based off the metadata.