[electrum] use GENERATOR and CURVE_ORDER from ecc module

Summary:
This replaces instances of ecdsa.ecdsa.generator_secp256k1 (instance of ecdsa.ellipticcurve.Point) with a ECPubkey instance.

After this diff, the only modules still importing ecdsa are:

• ecc.py
• ecc_fast.py
• schnorr.py (the pure-python fallback code)
• digitalbitbox.py

The plan is to now make ecc*.py and schnorr.py have a hard dependency on libsecp256k1 and remove the ecdsa dependency altogether.

This concludes backport of https://github.com/spesmilo/electrum/commit/16e4827e8c6db833b0a969351b73eb34d489681f

Depends on D16681

Test Plan:
python test_runner.py

Activate Cash Fusion and wait for a succesfull fusion

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16682