Integration feature/aes-tweaks with current release/0.1.3alpha

[officialfrancismendoza]

There are significant merge conflicts + divergence between feature/aes-tweaks, which contain a lot of important API-level changes, and the current state of release/0.1.3alpha (which at the time of this writing, has a new aes-lowmemory crate (#98, #99) and CBC mode compatible with the aes-lowmemory derivative (#100, #101).

This branch refactors the current state of release/0.1.3alpha to be compatible with the new BlockCipherEncryptor / BlockCipherDecryptor split and BlockPermutation.

[officialfrancismendoza]

No merge conflicts detected (although certainly a huge diff); rebuilding CFB mode off of this branch, which is was reconstituted on #111 and now on #113