[electrum] public_key_from_private_key return bytes

Summary:
Depends on D14567

The next diff will make most new hex conversions unneccessary by storing keypairs as bytes

Test Plan: python test_runner.py

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D14568