[electrum] don't use ecdsa for basic bytes <-> int conversion

Summary:
There are python builtin methods for that.

It is also a good opportunity to remove the misleading Python 2 era _to_string terminology when the output is actually bytes.

Partial backport of electrum#5947
https://github.com/spesmilo/electrum/pull/5947/commits/2cf213552822b0feda4775ed94719679bcb1efb4

Depends on D16665

Test Plan: python test_runner.py

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D16675