[backport] 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 PR14007:

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.

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