Make AES-NI optional

[tholop]

The current version of Sycret has a hard dependency on the aesni crate, which uses Intel's AES-NI instruction set. It appears to cause issues for some architectures which don't have access to this instruction set.

As suggested in the aesni crate documentation, we could fix this by:

• Checking whether AES-NI is available at runtime, e.g., with Cupid
• Fall back on a (slower) software implementation of AES if AES-NI is unavailable (e.g. the aes crate).