Add AES256 support and test vectors

[aronrosenberg]

Add support for the new AES256 SHA512 based cipher suites.

[bifurcation]

Couple of minor things that I will commit and then merge. I will file a follow-on PR to address some bigger comments:

1. This doesn't actually compile, due to a missing use
2. I think what we actually want here is an option that will only print the AES-256 test vectors, since that's what you'll want in the AES-256 extension spec. So I'll add command-line arguments to the executable so that by default, it prints the RFC test vectors, and with the option it prints the AES-256 ones. Something like

cargo run --example test_vectors # RFC test vectors
cargo run --example test_vectors --aes256ctr # RFC test vectors + AES-256
cargo run --example test_vectors --aes256ctr --only # Only AES-256

[bifurcation]

This doesn't need to be in a different module.

[bifurcation]

Also, CI is broken because the i-d-template tools are out of date. Separate issue.