Make isEmpty work for Sets

[lukelafountaine]

isEmpty doesn't currently work for Sets.

For example:

> let a = new Set([ 1, 2, 3, 4, 5 ]);
> isEmpty(a)
true

I had to hunt around to find the issue in my code and it came back to this. Seems like we have been using sets a bit more recently, so it would be nice to support it in the toolbox.