Nettle: Moved modes CTR and CCM to submodules.
CTR is now a submodule to Nettle.BlockCipher, and
CCM and CCM8 are submodules to Nettle.BlockCipher16.
Added fallback C-implementation of CTR if the Nettle library lacks one.
All common modes should now be submodules.
Crypto.CCM: Added digest_size().
Also adds some related AutoDoc mk II markup.
Crypto.CCM: Added Counter with CBC-MAC mode.
This cipher mode is specified in NIST Special Publication 800-38C.