Faster ed25519 blinding

Thanks to Paul ed25519 blinding is now fully two orders of magnitude faster!

  https://github.com/pyca/cryptography/issues/5068

Replaced slow_ed25519.py with an optimized implementation from...

  https://github.com/pyca/ed25519/

This changes the runtime of test_blinding as follows:

  Python 2.7: 2.25s => 20 ms
  Python 3.5: 1.83s => 19 ms