llcrypto: Implement ed25519_to_curve25519_private conversion.
In ArtiNativeKeyStore
, private keys are stored in OpenSSH format.
However, ssh-key
(the crate we use for parsing OpenSSH keys) doesn't
support x25519 keys. As a workaround, this type of key will stored
as ed25519 and converted to x25519 upon retrieval.
This commit implements the convert_ed25519_to_curve25519_private
conversion function (needed by ArtiNativeKeyStore
to support x25519
keys).
Part of #900 (closed)