Investigate libraries for Block cipher traits

[tuxxy]

This library could be improved significantly by using some pre-existing crypto traits for block ciphers.

I'll get around to it when I get around to it.

[tuxxy]

This should actually use the traits in https://github.com/RustCrypto/traits, specifically these for block ciphers: https://github.com/RustCrypto/traits/blob/master/block-cipher/src/lib.rs