The `convert_curve25519_to_ed25519_private` and
`convert_ed25519_to_curve25519_private` functions are now exported by
`tor-llcrypto` if the `cvt-25519` feature is enabled.
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
Gabriela Moldovan
Emil Engler
Nick Mathewson
dagon