Implement a key store

[rbost]

Although keys can be safely generated inside the library, they cannot be extracted (this is done on purpose), and hence cannot be stored. This is a problem when building real applications that need to be shut down and later restarted.

We must implement a way to store the keys in a file/stream that can be safely exposed. This in particular means encrypting this stream using authenticated encryption, with a key derived from a password.