Add implementation of the Argon2 password hash.

This is going to be used in the new version of the PPK file format. It
was the winner of the Password Hashing Context, which I think makes it
a reasonable choice.

Argon2 comes in three flavours: one with no data dependency in its
memory addressing, one with _deliberate_ data dependency (intended to
serialise computation, to hinder parallel brute-forcing), and a hybrid
form that starts off data-independent and then switches over to the
dependent version once the sensitive input data has been adequately
mixed around. I test all three in the test suite; the side-channel
tester can only expect Argon2i to pass; and, following the spec's
recommendation, I'll be using Argon2id for the actual key file
encryption.