[electrum] cleanup point_to_ser and stop leaking ecdsa implementation details for ECPubkey

Summary:
Better variable names, support Tuple[int, int] as input

User of ECPubkey should not have to know anything about the ecdsa library, so ECPubkey.point() now returns an int 2-tuple and ECPubkey.from_point() accepts such a tuple.

Depends on D16349

This is split out of D16301

Test Plan: python test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16350