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)
There are certainly things we _might_ want to do - e..g, represent an array of enums as a bit field or something. I think just because we can't think of something now doesn't mean we won't think of something in the future. In particular in this case, because even if we take the rest of this RFC I see essentially no downside in having to annotate bool-like enums with C-repr