added toArrayOf and toArrayOfOn

[dwhitney]

What does this pull request do?

I've added two functions Data.Lens.Fold (toArrayOf, toArrayOfOn) since Array is a fairly common data structure compared to List, which is already supported

Where should the reviewer start?

The functions are in Fold.purs and then re-exported in Data.Lens

How should this be manually tested?

I added an example in test/Main.purs that combines a couple of Prisms with traversed

[thomashoneyman]

Thanks, @dwhitney!