Nettle.BlockCipher: Inherit __builtin.Nettle.BlockCipher.

It is now possible to add generic stuff to all block ciphers by
adding it to __builtin.Nettle.BlockCipher.

NB: Requires cipher.cmod to be processed by a recent precompiler.
__builtin.Nettle.Cipher: Stonger type for name().

The function name() is now required to return a 7-bit string.
Updated to tzdata2014e.