[electrum] ecc: use libsecp256k1 for sign/verify/mul/add

Summary:
Make libsecp256k1 a mandatory dependency, drop the fallback pure-python code that was only used when running from sources without libsecp256k1 compiled.

Hard fail is there is any import error or unavailable symbol in secp256k1 (lib compiled without a required module, lib from Bitcoin Core...)

This is a partial backport of electrum#5947
https://github.com/spesmilo/electrum/pull/5947/commits/ad408ea832c55458b730945832c826894dfa9386

Depends on D16687

Test Plan: python test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16689