[electrum] move-only: extract more crypto code from bitcoin.py into ecc.py

Summary:
This is ecc.py move-only part of the original commit.

Inspired by https://github.com/spesmilo/electrum/commit/16e4827e8c6db833b0a969351b73eb34d489681f

Depends on D16303

Test Plan:
python test_runner.py

Run the application, test basic features ./electrum-abc

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16278