Add slidingVectorWindow

Hi Michael,

I probably should have checked first, but I only noticed that you started working on the same functionality when I wanted to set up a pull request. :)

My code seems to have turned out to be quite a bit simpler than yours, so I decided to show it anyway. Of course, I was only going for amortized constant time, and your code is real constant time. But, it would be easy to keep the simpler structure and still do the proper constant time at the cost of being very slightly inefficient.

Anyway, feel free to ignore this! Or if you'd like to, I'll clean up my solution and make it in a proper pull request. :)

Mihaly