[#161] refactoring `expression_hasher.py`

[angeloprobst]

Progress on ticket #161

Refactors expression_hasher.py:

1. Applies black formatting.
2. Ensures lines width at max 100 chars.
3. Adds and fixes type annotations, mainly returns.
4. Applies new standard of type annotations (i.e. dict[A, B] | None instead of Optional[Dict[A, B]]).
5. Adds and/or fixes docstrings.
6. Ensures the file passes green through pylint.
7. Fixes some typos.
8. Left some TODO comments for future evaluation.