Implement AES-CCM with SSSE3.

Split aes_impl declarations out into aes_impl.h.

This will make it less painful to add more operations to struct
aes_impl without having to recompile everything that just uses the
block cipher directly or similar.

New permutation-based AES implementation using SSSE3.

This covers a lot of CPUs -- particularly lower-end CPUs over the
past decade which lack AES-NI.

Derived from Mike Hamburg's public domain vpaes software; see
<https://crypto.stanford.edu/vpaes/> for details.