Provide constructors for keys and authenticators from string representation

[hannesm]

As mentioned in mirage/ocaml-git#482 both private keys and authenticators should be constructable from a string easily (used as command-line arguments to programs):

• the authenticator should specify type and fingerprint (or public key)
• the private key should specify type and seed (or private key)

This should be done before the next release. The current state is Keys.auhenticator_of_string and Keys.of_seed.

[hannesm]

That's done since 0.0.5.