Implement AES-XTS mode

XTS mode is fully known as "xor-encrypt-xor with ciphertext-stealing".
This is the generalization of the XEX mode.
This implementation is limited to an 8-bits (1 byte) boundary, which
doesn't seem to be what was thought considering some test vectors [1].

This commit comes with tests, extracted from [1], and benchmarks.
Although, benchmarks aren't really nice here, as they work with a buffer
of a multiple of 16 bytes, which isn't a challenge for XTS compared to
XEX.

[1] http://csrc.nist.gov/groups/STM/cavp/documents/aes/XTSTestVectors.zip

include/mbedtls/config.h

@@ -508,6 +508,14 @@
  */
 #define MBEDTLS_CIPHER_MODE_XEX
 
+/**
+ * \def MBEDTLS_CIPHER_MODE_XTS
+ *
+ * Enable Xor-encrypt-xor with ciphertext stealing mode (XTS) for symmetric
+ * ciphers.
+ */
+#define MBEDTLS_CIPHER_MODE_XTS
+
 /**
  * \def MBEDTLS_CIPHER_MODE_OFB
  *