initially I wanted to fix an issue in another project with diffs of `SerializeableDataclass`es being incorrectly non-empty when some elements were dicts. I haven't been able to reproduce this, but now there are a lot more tests (2331082).

Notably, (1de928d) also fixes an issue where `__eq__` would be overwritten by the `@serializable_dataclass` decorator, which is now fixed (but I forgot to write tests for this).

Commits:

- make `on_typecheck_mismatch` actually work (1de928d)
- add some todos in docstrings (d242c59)
- added a whole lot of serializable dataclass tests (2331082)
- don't ship old WIP code (41c57c6)
- change the _wip directory we ignore on format checks (adf4971)
- format (e8fea49)
- update version to 0.6.16 and deps (e3b26ab)
- rebuild docs (4f7557c)