Fully type module then mark as `py.typed`

This is an issue to track `jaraco.structures` being marked as `py.typed`.
I'd rather not lie about the typed state of a library and wait until all its public members are fully annotated (or at least to a "best effort") before being marked as `py.typed`.

Refs:
- https://github.com/jaraco/skeleton/issues/98
- https://github.com/jaraco/skeleton/pull/136#discussion_r1714057809
- https://github.com/jaraco/skeleton/issues/143

Start with the issue body and its three linked skeleton references, then inspect jaraco.structures' public members to identify the remaining typing work. Done means the public API is fully or best-effort annotated and the package is marked py.typed without overstating its typed status.

Written by the indexing model from the issue text.