Rational numbers wants an approaches and better instructions

[cmcaine]

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