crypto: Improved AES performance

attic.crypto now uses the OpenSSL EVP API for AES encryption.
This API uses AES-NI instructions when available resulting in
a significant AES encryption performance improvement:

Before: 80MiB/s
After: 1931MiB/s

Message size: 64kiB
CPU: Intel(R) Core(TM) i5-3320M CPU @ 2.60GHz