Made blockalgo.py more PEP-8 compliant

The second parameter of the _GHASH constructor
is now the length of the block (block_size)
and not the full module.
Add encrypt_and_digest() and decrypt_and_verify()

This patch adds encrypt_and_digest() and decrypt_and_verify()
methods to a cipher object.

In most cases they are just shortcuts to the existing functions.
For SIV mode, decrypt_and_verify() replaces decrypt().