Reformat AES CTR vectors to use NIST loader & document their source

This is a reformat of the AES CTR vectors from a format loaded via `load_openssl_vectors` into the more standard `load_nist_vectors` format we typically use.

This change is to simplify an upcoming CAST5-CTR vector generation PR. I will also be submitting one more PR after this for reformatting the Camellia CBC/CFB/OFB vectors into NIST loader form so we can remove the `load_openssl_vectors` code entirely.