Add friendlier solutions to the union exercises, some cosmetic edits

The "cosmetic edits" were basically rearranging things in the files so that similar things are in the same places. I also did this to the text, so things didn't seem quite as disjointed.

I commented out @wei2912's explanation of intersects, because i didn't think it was fleshed out enough. I added a slightly more fleshed out explanation, I will expand on it later.