Supports calculating CIDR unions, differences, and complements. The rest of the existing set operations don't really make sense for CIDRs since the only way they can overlap is if one fully contains the other (or they are equivalent).
I figured a binary tree-like data structure would be the most memory efficient way to represent this, but I'm super open to any feedback especially if there's a well established better way to do this.
Play with it on the command line if you have rust installed: `cargo install fcidr`!