Refactor crypto.SavePrivateKey and crypto.Link

[woutslakhorst]

Both are mainly used for tests and are there for either convenience or for a test which uses predefined keys. This is not really needed.

crypto.SavePrivateKey is also used to migrate your keys from one backend to another. The goal is to use a backend that doesn't support exporting private keys (or you do it outside of nuts and change the SQL db to map accordingly)