rfctr: prepare to add orig_elements serde (#2668)
**Summary**
The serialization and deserialization (serde) of
`metadata.orig_elements` will be located in `unstructured.staging.base`
alongside `elements_to_json()` and other existing serde functions.
Improve the typing, readability, and structure of that module before
adding the new serde functions for `metadata.orig_elements`.
**Reviewers:** The commits are well-groomed and are probably quicker to
review commit-by-commit than as all files-changed at once.
