Add sr25519

[conr2d]

Schnorrkel/Ristretto x25519 ("sr25519") is a schnorr signature scheme on Ristretto compressed ed25519 points used in Substrate framework.

You can find more information:

• Schnorrkel: https://github.com/w3f/schnorrkel
• Ristretto: https://ristretto.group/ristretto.html