Implement XTS block cipher mode

Hello,

The attached patch implements the XTS block cipher mode, as specified in IEEE P1619. The interface is split into a generic pair of functions for encryption and decryption and additional AES-128/AES-256 variants.

The function signatures follows the same pattern used by other block- cipher modes like ctr, cfb, ccm, etc...

Basic tests using a small selection of NIST CAVS vectors are provided.

XTS is use in several disk encryption algorithms and programs because it allows to use a block cipher even when the input length is not a perfect multiple of the block cipher length by using ciphertext stealing.

Thanks to Daiki Ueno for initial review.

Feedback is appreciated.

Simo.