Expose c_zmq_curve_keypair via curveKeyPair

Can I please have something like this?

I'd like to generate client keys on the fly, by calling this:
http://api.zeromq.org/4-0:zmq-curve-keypair

Crypto support currently works perfectly for me, by the way.

This pull request works, I didn't know where to put things though and I've only been using haskell for a month or two now. So. Careful.