Expose interface for secret key to public key conversion.

[jakubtrnka]

I implemented a simple certificate infrastructure.
The server is sending certificate with its static public key signed by some authority to a client.
The server on startup reads file with the certificate and its secret static key.

I want to implement validation of the server's secret key against the public key in the certificate so that I can tell "This secret key belongs to that public key".

There is, no api for this. I need to take the cryptographic algorithm and do the check manually, bypassing the abstraction layers that are hiding details about the underlying algorithms.

I imagine something like adding a method to the Builder: Builder::build_keypair(&self) -> Result<Keypair, Error> that doesn't generate new keypair but instead takes local private key that was set previously with method local_private_key and calculates corresponding public key. Perhaps even exploit generate_key_pair for that purpose in case user sets explicitly private key.

[jakubtrnka]

I'm willing to implement this if the idea gets positive acknowledgement.