Scalar fixed time AES core transform

This is a straight conversion of the aes_ti.c & aes.h code from the linux kernel.

WARNING

This should be used with a good mode (GCM, EME, XTS, etc). Aes here only implements encryption/decryption of a single block. It is essentially ECB only.

In most cases using a complete cryptosystem library like NACL, sodium, and others will be a better choice. These try to avoid various missteps that can compromise the intended use of this block cipher.