Pure python EC

Summary:

This removes the dependency on OpenSSL for the interaction tests, by
providing a pure-Python
toy implementation of secp256k1.

Backport of core PR15826.

Test Plan:

ninja check-functional-extended

*The following depends on D14007:*
Build for windows, then:

• Install python if needed and add it to your path
• If you copy the project to some place, you need at least the src,

test, share and your build directories.

• Copy the test_runner.py to the build directory to replace the symlink
• Update the config.ini file to match you paths

Then in a powershell:

$env:PYTHONIOENCODING="utf-8"
python .\test\functional\test_runner.py --force

You should note that only a few tests remain red, in particular all the
schnorr related tests since they still use openssl.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5965