Make it easier to query the public key for a saved private key

[robin-nitrokey]

To get the public key of a stored private key, I currently have to:

• derive the public key
• serialize the derived key
• delete the derived key (even if the previous step failed)

This is non-intuitive and may fill up the volatile storage if the last step is omitted or not executed on all return paths. I think the Trussed API should make it easier to perform this operation correctly.

Improvement ideas:

• add a helper function for public key serialization
• make it possible to call serialize on a private key to get the public key