Switch `CompletionContext` to `dataclass`, use `cached_property` Data class has narrower API than named tuple (does not expose item getters) which means it is less likely that the downstream code would be broken in the future if we introduce new attributes. Also, compared to other places where memory-efficient named tuple is used, `CompletionContext` is only created once per completion and thus does not require as low overhead. For the same reason we can forgo slightly more memory-efficient `@property @cache` stack and just use `@cached_property`.