Add public functions to inspect dataclass parameters and methods

dataclasses were originally intended as boilerplate writers but due to their structure a number of tools special case them and in doing so currently rely on either internal attributes or external observations. I think we should acknowledge this and provide correct public functions to access the relevant parts of dataclasses. The idea is to expose the relevant details without requiring the use of internal names or really specific internal details (see the first example). Proposal: Add is_dat...