Clean up after building a version to save space
Talked to someone with not a whole lot of space left on their device that wanted to give Nim a try today. Checked how much `choosenim` was using on my system and it showed a pretty steep 4.1G of used space. I've got about 9 versions installed, but 0.19.0 alone is almost half a gig large. The main culprit for each version appears to be the c_code folder. The downloads folder is another half gig combined. It would be nice if choosenim could clean up these files that are no longer used for anything so that it didn't hog as much space.