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Clarify how we derive ed25519 for cross-certification.

Nick Mathewson requested to merge nickm/torspec:curve25519_to_ed25519_clarification into main

The descriptor format uses a curve25519->ed25519 conversion algorithm to cross-certify descriptors with their ntor onion keys.

This patch clarifies two aspects of the algorithm:

  1. When deriving a private key, how to derive the part of the private key that isn't a point on the curve.

  2. That there are two algorithms here, one for private->private and one for public->public.

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