Represent equivalent `enum`s the same as `bool`, make `bool` an `enum`

[CLICKME](https://github.com/glaebhoerl/rfcs/blob/make-bool-an-enum/active/0000-make-bool-an-enum.md)
For 98.5% of our uses a variable in the prelude is effectively a keyword, so this is not much of a benefit. Certainly most tools (syntax highlighting, etc.) will continue to consider bool/true/false as keywords and using them yourself will not be much fun.