[electrum] cleanup ser_to_point

Summary:
Use better variable names, split the function from ser_to_point(bytes) -> Point into ser_to_coordinates(bytes) -> (int, int)] and ser_to_point(bytes) -> Point.

This will allow us in future commits to remove some ecdsa import in other modules, to keep implementation details contained in ecc.py.

Depends on D16347

This is split out of D16301

Test Plan: python test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16348