Exposing np.select?

Should we support np.select? It's a bit controversial because one could chain np.where to emulate np.select, but I thought this would be a cleaner way to implement inducer/meshmode#355.

I think a canned implementation in terms of wheres makes sense, though perhaps not with pressing urgency. The main reason to me would be that select could boil down to $\log(n)$ actually executed ternaries as opposed to $n$.