This adds an independent Schnorr implementation like the functional test
framework's key.py. No new dependencies as it also runs on OpenSSL.

This will be needed so that we can produce real transaction signatures for
activation testing.

Includes a basic self-test if you execute the file, to make sure it
produces identical sigs as the implementation in libsecp256k1.

Alternative to D2487

includes self-test if you run the python file