WIP: numba arithmetic

I suggest that ast construction happens in another function than `compute_up`, kind of like is done for pyfunc printing https://github.com/ContinuumIO/blaze/blob/master/blaze/compute/pyfunc.py.  This should be nicely separable and testable from direct computation.  It also won't be tied to numpy.

Then, in `compute_up(Broadcast, np.ndarray, ...)` we call that function hand the result off to numba and then execute that.