This adds an implementation of the SM4 block cipher, and operations
to encrypt and decrypt using it with CBC,ECB,CFB,OFB,CTR modes.
Also, a "no padding" option is added for AES,DES,3DES and SM4
decryption in ECB/CBC modes. This variant does not attempt to
validate the last block as being PKCS#7 padded.
This is useful, both since other padding schemes exist, and also
for decrypting data where the final block is missing.
