core/general: add assert_never + typing annotations for dataset

my/instagram/gdpr.py

@@ -56,7 +56,7 @@ def _decode(s: str) -> str:
 
 import json
 from typing import Union
-from ..core.error import Res
+from ..core import Res, assert_never
 def _entities() -> Iterator[Res[Union[User, _Message]]]:
     from ..core.kompress import ZipPath
     last = ZipPath(max(inputs()))
@@ -165,4 +165,4 @@ def messages() -> Iterator[Res[Message]]:
                 user=user,
             )
             continue
-        assert False, type(x)  # should not happen
+        assert_never(x)