[electrum] move-only: split part of bitcoin.py into crypto.py

Summary:
The bitcoin.py module is huge and deals with a variety of unrelated things. This is a small step towards improving the situation.

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

Test Plan:
python test_runner.py

Run the application, do a few basic actions (send, receive txs) to check that core features are not broken

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16277