[electrum] move serialize.py out of avalanche

Summary:
This is useful for the rest of the codebase too.
Next I will move COutPoint to transaction.py, and it needs this module.

Depends on D14424

Test Plan: python test_runner.py

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D14432