Rational numbers wants an approaches and better instructions

Instructions are too long and useful info is buried in it. It's also formatted badly by default because inline code blows up the line-height, at least sometimes.

The problem has a bunch of subtleties, so an approaches file would be appropriate.

Important to highlight:

1. Default methods save you effort if you let them
2. Representing infinity (a good representation is easier for users to understand; makes default equality work; makes ordering easier)
3. Explanation of less-than method
4. Dangers of big products and how to reduce those dangers
5. Secret constructors for speed