Implement CMAC

[a-dma]

I was playing around with CMAC and I thought I'd contribute back what code I have.

I'm still learning rust, so the code is by no means perfect. Also, given the lack of documentation there was a bit of guesswork to find things that were already implemented.

A couple of possible (but for me tricky) improvements are:

• Use a FixedBuffer for storing the MAC result and take advantage of StandardPadding;
• Try to shoehorn in a CbcEncryptor with an empty IV rather than doing the XOR'ing manually.

[flybayer]

Any chance this will be merged to master soon?

[flybayer]

@DaGenix is anything besides the failing build preventing this from being merged?

[tvladyslav]

@flybayer last commit was merged 7 month ago. It smells like a dead repo. I thought about making one small PR, but now I have doubts...