[electrum] implement size computation for TxOutput

Summary:
This will be useful for making the coinchooser not produce transactions that are larger than the 100kB relay policy.

Depends on D14431

Test Plan: python test_runner.py

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D14424