isna() for scalar case

[alyst]

It significantly reduces memory allocation overhead in DataFrames for me on 0.4.

[johnmyleswhite]

It seems like we're probably being too liberal with indexing if this is needed since I would have thought the other cases would handle this.

[simonster]

Most likely there is an issue with Compat's @nsplat. In any case once we can drop 0.3 support we should replace as much of the Cartesian use as possible with generic functions.