According to https://rustsec.org/advisories/RUSTSEC-2016-0005.html,
rust-crypto is unmaintained.
Crates depending on rust-crypto should be ported to other crates.
This port replaces rust-crypto with the sha2 implementation of ring,
as fido2luks already depends on it via ctap_hmac. Note that it uses
an old version of ring, so I used the same version, here.
