improve API (make it harder to use wrong)

[hannesm]

as discussed with @emillon

• remove Authenticator.null
• make Authenticator.t abstract (and provide a constructor) <- more easily discover Authenticator.t uses
• use time:(unit -> Ptime.t option) ((a) being explicit about time - (b) rename time to something more sensible - (c) also use a function to avoid the timestamp used at Authenticator creation time vs actual authentication)
• use host:[`host] Domain_name.t option as well

[hannesm]

• provide a pp for Certifiacte.Host_set.t (maybe it should be X509.Host_set -- i.e. no Certificate, since it is as well used by Signing_request)

[hannesm]

see #131