Compare area on screen to histogram

[emiln]

In order to write logic for visual content on the screen, a simple start would be the ability to express a condition like:

Take a histogram of the area [x, y, width, height]. Is this within [distance] of [other histogram]?

As histograms were already implemented with #11 this is actually mostly done. A few tasks remain:

• [ ] Allow script authors to write literal histograms. Perhaps [:histogram 0xffff...].
• [ ] Allow script authors to generate a histogram of a portion of the screen. Perhaps [:histogram-at x y width height].