[electrum] use libsecp256k1 for signature conversions

Summary:
This is a partial backport of electrum#5947
https://github.com/spesmilo/electrum/pull/5947/commits/ab0c70e2918778a44a33089ab86729c6b25a9556

Depends on D16689

Test Plan: python test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16690