Support the UMAC MAC

[glguy]

It would be neat to incorporate support for UMAC into cryptonite.

I've written a binding to a C implementation of this algorithm (as used in openssh) that I'd be happy to have incorporated into cryptonite with whatever changes were necessary. I've specialized the C implementation to the two MAC sizes that I needed for SSH (64 and 128) but the algorithm supports more sizes than that (and it might be desirable for cryptonite to do so).

Currently the implementation compiles against a C implementation of AES. I imagine that it'd be desirable to have it work against the existing implementation of AES in cryptonite if incorporated.

I'm opening this issue to start discussion on the topic.

Specification of UMAC:
https://www.ietf.org/rfc/rfc4418.txt

Current implementation:
https://github.com/glguy/ssh-hans/blob/eric/src/Crypto/MAC/UMAC.hs